A genetic explanation for the rising incidence of type 1 diabetes, a polygenic disease.

We had earlier hypothesized, if parents originated from previously isolated populations that had selected against different critical susceptibility genes for a polygenic disease, their offspring could have a greater risk of that disease than either parent. We therefore studied parents of patients with type 1 diabetes (T1D). We found that parents who transmitted HLA-DR3 to HLA-DR3/DR4 patients had different HLA-A allele frequencies on the non-transmitted HLA haplotype than HLA-DR4-transmitters. HLA-DR3-positive parents also had different insulin (INS) gene allele frequencies than HLA-DR4-positive parents. Parent pairs of patients had greater self-reported ethnicity disparity than parent pairs in control families. Although there was an excess of HLA-DR3/DR4 heterozygotes among type 1 diabetes patients, there were significantly fewer HLA-DR3/DR4 heterozygous parents of patients than expected. These findings are consistent with HLA-DR and INS VNTR alleles marking both disease susceptibility and separate Caucasian parental subpopulations. Our hypothesis thus explains some seemingly disconnected puzzling phenomena, including (1) the rising world-wide incidence of T1D, (2) the excess of HLA-DR3/DR4 heterozygotes among patients, (3) the changing frequency of HLA-DR3/DR4 heterozygotes and of susceptibility alleles in general in patients over the past several decades, and (4) the association of INS alleles with specific HLA-DR alleles in patients with T1D.

Conserved extended haplotypes discriminate HLA-DR3-homozygous Basque patients with type 1 diabetes mellitus and celiac disease.

The major susceptibility locus for type 1 diabetes mellitus (T1D) maps to the human lymphocyte antigen (HLA) class II region in the major histocompatibility complex on chromosome 6p21. In southern European populations, like the Basques, the greatest risk to T1D is associated with DR3 homo- and heterozygosity and is comparable to that of DR3/DR4, the highest risk genotype in northern European populations. Celiac disease (CD) is another DR3-associated autoimmune disorder showing certain overlap with T1D that has been explained by the involvement of common genetic determinants, a situation more frequent in DR3-rich populations, like the Basques. As both T1D- and CD-associated HLA alleles are part of conserved extended haplotypes (CEH), we compared DR3-homozygous T1D and CD patients to determine whether CEHs were equally distributed between both disorders or there was a differential contribution of different haplotypes. We observed a very pronounced distribution bias (P<10(-5)) of the two major DR3 CEHs, with DR3-B18 predominating in T1D and DR3-B8 in CD. Additionally, high-density single nucleotide polymorphism (SNP) analysis of the complete CEH [A*30-B*18-MICA*4-F1C30-DRB1*0301-DQB1*0201-DPB1*0202] revealed extraordinary conservation throughout the 4.9 Mbp analyzed supporting the existence of additional diabetogenic variants (other than HLA-DRB1*0301-DQB1*0201), conserved within the DR3-B18 CEH (but not in other DR3 haplotypes) that could explain its enhanced diabetogenicity.

Mendelian inheritance of polygenic diseases: a hypothetical basis for increasing incidence.

The incidence of common polygenic diseases, such as type 1 diabetes, bronchial asthma, and gluten-sensitive enteropathy, is increasing. Although this is usually attributed to environmental factors, it is possible that this rising incidence also has a genetic basis. The hypothesis is put forth that, in the past, these diseases, with their increased morbidity and mortality, were selected against. In contrast to monogenic diseases, the incidence of polygenic diseases can be reduced by selection against susceptibility alleles of any of the genetic loci necessary for disease to occur. In different isolated populations, different disease susceptibility loci may have been selected against. Parents who derive from different isolated populations in which there are inversely different susceptibility allele frequencies because of selection or genetic drift, would be expected to have offspring with an increased risk for that polygenic disease. It is shown mathematically that the incidence of a hypothetical polygenic disease increases under these circumstances. The increased risk in these offspring results from a kind of genetic complementation in which they have inherited a more complete set of susceptibility alleles at all susceptibility loci than is carried by either of their parents. Hallmarks of this hypothesized phenomenon would be increased heterozygosity for specific population markers (whether susceptibility alleles or not) among the disease-affected offspring and a paucity of such heterozygotes among their parents. The parents and patients would also be expected to give more evidence of ethnic or subethnic disparity than that observed in controls.

Inheritable variable sizes of DNA stretches in the human MHC: conserved extended haplotypes and their fragments or blocks.

The difference in sizes of conserved stretches of DNA sequence within the major histocompatibility complex (MHC) in human individuals constitutes an underappreciated genetic diversity that has many practical implications. We developed a model to describe the variable sizes of stretches of conserved DNA in the MHC using the known frequencies of four different kinds of small (< 0.2 Mb) blocks of relatively conserved DNA sequence: HLA-Cw/B; TNF; complotype; and HLA-DR/DQ. Each of these small blocks is composed of two or more alleles of closely linked loci inherited as one genetic unit. We updated the concept of the conserved extended haplotype (CEH) using HLA allele identification and TNF microsatellites to show that specific combinations of the four blocks form single genetic units (>/= 1.5 Mb) with a total haplotype frequency in the Caucasian population of 0.30. Some CEHs extend to the HLA-A and -DPB1 loci forming fixed genetic units of up to at least 3.2 Mb of DNA. Finally, intermediate fragments of CEHs also exist, which are, nevertheless, larger than any of the four small blocks. This complexity of genetic fixity at various levels should be taken into account in studies of genetic disease association, immune response control, and human diversity. This knowledge could also be used for matching CEHs and their fragments for patients undergoing allotransplantation.

Polymorphism of the human retinoid X receptor beta and linkage disequilibrium with HLA-DPB1.

The human retinoid X receptor beta (RXRB) gene is localized in the major histocompatibility complex (MHC) region between DPB1 and RING2. The RXRB gene sequence reported by different investigators suggests that the gene may be polymorphic. In this study, we confirmed one polymorphism by sequencing genomic DNA from four Caucasian individuals. We also developed a restriction fragment length polymorphism (RFLP) analysis to detect this specific polymorphism. Linkage analysis studies between RXRB alleles and a number of HLA markers showed significant linkage disequilibrium between RXRB*T and HLA-DPB1*0401.

Hepatitis B surface antigen- and tetanus toxoid-specific clonal expansion of CD4+ cells in vitro determined by TCRBV CDR3 length and nucleotide sequence.

We demonstrate activation of primary human TCRBV-specific CD4+ cells in vitro towards hepatitis B surface antigen (HBsAg) and tetanus toxoid (TT) without the use of cell lines, clones or added cytokines. By multiplex PCR analysis and spectratyping, antigen-activated cells exhibited clonal T cell receptor expansion within specific and limited TCRBV families. The expanded CD4+ T cells were CD45RO. Three of four unrelated HBsAg responders showed CD4+ expansion within the TCRBV16 family. The response comprised predominantly single CDR3 sequences in all three donors and was completely monoclonal in one of them. However, the CDR3 lengths and sequences differed among the responders. Clonality induced by HBsAg in TCRBV16 was specific, reproducible and distinct from that induced by TT in terms of sequence, nucleotide addition and diversity (BD) or junctional (BJ) element usage. Thus, for the first time, we show monoclonal or oligoclonal expansion of primary human CD4- peripheral blood mononuclear cells (PBMC) in vitro in response to nominal protein antigen without manipulations utilizing exogenous IL-2. The ability to induce monoclonal/ oligoclonal responses to HBsAg now permits motif identification studies for determining the T cell role in nonresponsiveness to the HBsAg vaccine.

HLA-Cw alleles associated with HLA extended haplotypes and C2 deficiency.

There are four MHC-linked complement genes, BF, C2, C4A and C4B, that are inherited as single DNA units, known as complotypes. Extended haplotypes were initially defined by studying the distribution of complotypes in relation to HLA-B and HLA-DR loci in Caucasian families. In order to analyze the distribution of HLA-Cw alleles in relation to extended haplotypes, we studied a large panel of MHC homozygous and heterozygous cell lines representing previously described Caucasian-derived extended haplotypes and 14 patients with complete C2 deficiency. HLA alleles were assigned using sequence-specific oligonucleotide probe hybridization (SSOP). Family analysis served to assign haplotypes for heterozygous samples. We found distinctive HLA-Cw alleles for each independent extended haplotype. Their association in each instance was statistically significant. All patients with C2 deficiency carrying the haplotype [HLA-B18, S042, DR2] were associated with HLA-Cw*1203. These conserved allelic combinations may become an important tool for the study of human evolution and may contribute to the expeditious selection of prospective donors in clinical transplantation.

Predictability of alloreactivity among unrelated individuals: role for HLA-DPB1.

We compared the mixed lymphocyte culture reaction (MLR-1) among unrelated individuals who are carriers of the extended haplotype [HLA-B8,SC01,DR3,DRB1*0301,DQB1*0201] on one chromosome and the generic specificity HLA-DR4 on the second chromosome. Genomic DNA samples from the same individuals were also analyzed for HLA-DRB1, DQB1 and DPB1 alleles by PCR and SSOPH typing and for DOB polymorphism by RFLP. HLA-DRB1 alleles, in paired MLR responses between unrelated individuals indicated that matching of HLA-DRB1 was a better predictor of non-reactivity than identity in HLA-DR generic types, (43% vs 22%). Moreover, 90% of the DRB1 matched pairs had nonreactive and weakly reactive MLR, whereas only 37% of DRB1 mismatched unrelated individuals gave weak or no reactions. Matching for HLA-DRB1 and HLA-DPB1 alleles eliminates a significant number of cell mixtures with MLR-I reactivity. Furthermore, some DPB1 mismatches, but not all, do not seem to elicit MLR-I reactivity.

HLA-DQB1*0602 is associated with dominant protection from diabetes even among islet cell antibody-positive first-degree relatives of patients with IDDM.

HLA-DQB1 alleles confer susceptibility and resistance to insulin-dependent diabetes mellitus (IDDM). We investigated whether the susceptibility alleles DQB1*0302 and DQB1*0201 affect progression to diabetes among islet cell antibody-positive (ICA+) first-degree relatives of IDDM patients and whether the protective allele DQB1*0602 can be found and is still protective among such relatives. We human leukocyte antigen-typed and periodically tested beta-cell function (first-phase insulin release [FPIR] during the intravenous glucose tolerance test) in 72 ICA+ relatives, of whom 30 became diabetic on follow-up (longest follow-up 12 years); 54 (75%) relatives carried DQB1*0302 and/or DQB1*0201. The frequency of DQB1*0302 and DQB1*0201 and of the high-risk genotype DQB1*0302/DQB1*0201 did not differ significantly between diabetic relatives and those remaining nondiabetic. On follow-up, progression to IDDM was not statistically different for relatives with or without the DQB1*0302/DQB1*0201 genotype. However, those relatives with the DQB1*0302/DQB1*0201 genotype had a tendency to develop diabetes at an earlier age (log-rank P = 0.02). We found DQB1*0602 in 8 of 72 (11.1%) ICA+ relatives. Relatives with DQB1*0602 did not develop diabetes or show any decline of FPIR versus 28 of 64 DQB1*0602- relatives who developed IDDM (log-rank P = 0.006; Wilcoxon's P = 0.02). The protective allele DQB1*0602 is found in ICA+ relatives who have minimal risk of progression to IDDM. Therefore, DQB1*0602 is associated with protection from IDDM both in population studies and among relatives with evidence of autoimmunity who should not enter prevention trials.

The paternally inherited insulin gene B allele (1,428 FokI site) confers protection from insulin-dependent diabetes in families.

Several polymorphisms of the insulin gene and its flanking regions (INS region) are in linkage disequilibrium and confer susceptibility to insulin-dependent diabetes (IDDM). We have analysed INS AA and AB-BB genotypes at the 1,428 FokI site (3' of the insulin gene) in 217 patients with IDDM, 402 non-diabetic first degree relatives negative for insulin (IAA) and islet cell autoantibodies (ICA), and 116 autoantibody positive (for ICA or IAA, or both) relatives of whom 39 became diabetic on follow-up. Most IDDM patients (83.4%, 181/217) had the AA genotype vs. 50% (25/50) of the controls (P < 10(-6)). Only 16.6% (36/217) of IDDM patients carried the AB genotype and none was BB homozygous, suggesting a protective effect of the B allele. By segregation analysis of the B allele in the IDDM offspring of informative families (only one AB parent) from the United States, the maternal B allele was inherited by 19/35 (54.2%) of the IDDM offspring. In contrast, only 4/26 (15.3%) of the IDDM offspring inherited the paternal B allele (P = 0.001), suggesting maternal imprinting of the INS region. Therefore, the INS B allele may be protective only when paternally inherited. Among the 39 of 116 autoantibody positive relatives who developed IDDM on follow-up, only five of them had the B allele. The frequency of the B allele in this group was much lower (12.8%, 5/39) than that observed in non-diabetic autoantibody positive relatives (32.5%, 25/77, P = 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)

Two subsets of HLA-DQA1 alleles mark phenotypic variation in levels of insulin autoantibodies in first degree relatives at risk for insulin-dependent diabetes.

Levels of insulin autoantibodies (IAA) vary among different first degree relatives of insulin-dependent diabetes mellitus patients, suggesting genetic regulation. We previously reported elevated IAA among DR4-positive at risk relatives. In this study, 72/82 at risk relatives were IAA positive, of whom 75% (54/72) carried DR4 versus 20% (2/10) of IAA-negative relatives (P = 0.0004). However, 69% (18/26) of DR4-negative relatives were IAA positive. Since DR4 did not account for all IAA positivity, we analyzed DQA1 and DQB1 alleles. Homozygosity for DQA1 alleles deriving from the evolutionary lineage 4 (*0401, *0501, *0601) was associated with low IAA levels, while lineage 1-3 alleles (*0101, *0102, *0103, *0201, *0301) correlated with higher levels. Most (93%, 65/70) relatives with lineage 1-3 alleles were IAA positive (mean = 360 +/- 63 SEM nU/ml). Only 7/12 relatives homozygous for lineage 4 alleles were IAA-positive, with lower levels than relatives with lineage 1-3 alleles (mean = 55 +/- 15 SEM nU/ml, P < 0.0001; 7/12 vs 65/70, P = 0.004). The amino acid sequences of lineage 1-3 alleles uniquely share glutamic acid (E) and phenylalanine (F) at positions 40 and 51 (EF alleles). Lineage 4 alleles have glycine (G) and leucine (L) at those positions (GL alleles). 90% (65/72) of IAA-positive relatives had an EF allele, while only 75% (54/72) had DR4 (P = 0.01). Homozygosity for GL alleles (often DQA1 *0501 on DR3 haplotypes) correlated with little or no humoral response to insulin. Thus, HLA-DQB1 GL alleles, or other genes on haplotypes (e.g., DR3) that carry these DQA1 alleles, may confer recessive low responsiveness to insulin.

Association of HLA-DQB1*0201 with stiff-man syndrome.

Stiff-man syndrome (SMS) is a rare disorder of the central nervous system of probable autoimmune origin. Patients with SMS often have other autoimmune diseases, in particular type I (insulin-dependent) diabetes mellitus (IDDM). Approximately 60% of patients with SMS have high titers of autoantibodies against the enzyme glutamic acid decarboxylase. Similar to SMS, the majority of patients with IDDM have autoantibodies against glutamic acid decarboxylase at or before diabetes onset, although usually at a lower titer and with a different reaction pattern than patients with SMS. To investigate the immunogenetic basis of SMS, we HLA-typed 18 patients with the disease. Seventy-two percent carried the DQB1*0201 allele (13 of 18, P = 0.02 vs. 18 of 48 controls), indicating that SMS is associated with this allele. DQB1*0201 is also a susceptibility allele for IDDM and other autoimmune diseases. Patients with SMS carried the IDDM-protective DQB1*0602 allele and other sequence-related DQB1*06 alleles with the same frequency observed in controls. In contrast, these alleles are rarely found in IDDM. Five of 8 (62.5%) SMS patients lacking a DQB1*06 allele were diabetic in contrast to only 2 of 10 (20%) with a DQB1*06 allele (P = 0.08), suggesting that the presence of DQB1*0602 or other DQB1*06 alleles may be associated with a reduced prevalence of diabetes among patients with SMS.

Characterization of type I complement C2 deficiency MHC haplotypes. Strong conservation of the complotype/HLA-B-region and absence of disease association due to linked class II genes.

Fourteen individuals with complete C2 deficiency from 11 families and 3 heterozygous C2-deficient individuals from two families were investigated. In all the 24 independent C2-deficient haplotypes, the complotype S042 was present and the majority (21/24) was [HLA-B18,S042,DR2]. All carried the type I C2 deficiency C2 pseudogene with its characteristic 28 bp deletion. All but two haplotypes had 10 AC/GT repeats in the TNF alpha microsatellite polymorphism and all but one of the haplotypes were identical at or near HLA-B as assessed by RFLP using BstEII digestion and two genomic probes, R5A and M20A, located 100 and 38 kb centromeric to HLA-B, respectively. The exceptional haplotype was HLA-B40 with four AC/GT repeats at TNF-alpha. Three of the haplotypes were not DR2 based on generic and sequence-specific oligonucleotide typing. Another four haplotypes showed different DO-variants detected by RFLP analysis using BglIIand Mspl digestion. Thus, the [HLA-B18,S042,DR2] haplotype appears to be more fixed in the region between the complement genes and the HLA-B locus (96%) than in the region between the complement genes and DR (88%) and DO loci (71%). Of the 14 individuals studied, six had SLE or SLE-like syndromes and six had a history of severe infections although two were apparently healthy. Three of the six SLE patients and two individuals with repeated infections were homozygous for [HLA-B18,S042,DR2] and also homozygous for DQB1*0602 and the common DO variant. Thus, MHC class II genes linked to the C2 pseudogene do not appear to determine different clinical consequences of C2 deficiency.

No independent association between HSP70 gene polymorphism and IDDM.

A role for heat shock proteins (HSPs) in autoimmunity has recently been suggested by several authors. Autoantibodies against HSPs have been associated with such autoimmune diseases as systemic lupus erythematosus, polymyositis, and the NOD mouse model of diabetes. Moreover, genes for the major 70,000-M(r) HSP (HSP70) are located within the MHC. To investigate a potential association of an HSP70-2 gene polymorphism with insulin-dependent diabetes mellitus (IDDM), we analyzed restriction-fragment-length polymorphism (RFLP) of this gene in 29 families with one or more member affected by IDDM. With the enzyme PstI, as reported previously, two HSP70-2 alleles of 8.5- and 9.0-kb were found. The 8.5-kb allele was found more frequently on diabetic haplotypes compared with control haplotypes (41 of 66 [62%] vs. 20 of 46 [43%], P = 0.03). This association was due to the conservation of alleles on extended haplotypes we previously reported to be associated with diabetes on initial analysis of families. Twenty-three of 26 diabetic DR3 haplotypes and 3 of 3 normal DR3 haplotypes and all instances of [HLA-B8, SC01, DR3] and [HLA-B18, F1C30, DR3] had the 8.5-kb allele, whereas 0 of 9 normal DR2 haplotypes and 0 of 2 diabetic DR2 haplotypes had the 8.5-kb allele (P = 8 x 10(-7) DR3 vs. DR2 haplotypes). The alleles were equally distributed among DR4 haplotypes.(ABSTRACT TRUNCATED AT 250 WORDS)

Predictability of alloreactivity among unrelated individuals.

Extended haplotypes are specific HLA-B, BF, C2, C4A, C4B and DR allelic combinations that occur at high frequencies and show positive linkage disequilibrium among these highly polymorphic MHC markers. About 30% of all normal caucasian haplotypes are extended, and the matching of two extended haplotypes in unrelated individuals has been shown to match for the determinants of primary mixed lymphocyte reactivity (MLR-I). In this work we report that the matching of one extended haplotype and a serologically defined HLA-DR generic type on the second chromosome in unrelated individuals is associated with the absence of mixed lymphocyte reactivity in 15 to 30% of the cases studied. Our results suggest that, for those individuals who carry either one or two extended haplotypes, it is relatively easy to identify an unrelated MLR-I-matched subject. However, for individuals lacking at least one extended haplotype, it should be difficult to find an MLR-I-matched unrelated subject.

Specific association of HLA-DR4 with increased prevalence and level of insulin autoantibodies in first-degree relatives of patients with type I diabetes.

First-degree relatives of patients with insulin-dependent (type I) diabetes (n = 264 from 106 families) were evaluated with HLA typing and determination of competitive insulin autoantibodies (CIAAs) and islet cell autoantibodies (ICAs). The levels of CIAAs in 30 relatives exceeded our upper limit of normal (greater than or equal to 39 nU/ml), and 30 had high-titer ICAs (greater than or equal to 40 Juvenile Diabetes Foundation units [JDF U]). Eleven of the HLA-typed relatives developed diabetes during follow-up. Twenty-three percent (28 of 123) of the relatives with at least one HLA-DR4 allele were CIAA+ (CIAA greater than or equal to 39 nU/ml) versus 4% (6 of 141) among DR4- relatives (P less than 0.0001). Twenty-one of 22 of the highest CIAA values were all in the DR4+ group (DR4+ vs. DR4-, P = 0.003, Wilcoxon's rank-sum test). HLA-DR3 did not correlate with the level of CIAAs, and neither DR3 nor DR4 correlated with titer of ICAs measured in JDF U. We conclude that, in first-degree relatives of patients with type I diabetes, there is a striking association with HLA-DR4 in both the prevalence of relatives exceeding the normal CIAA range and in the level of CIAAs. These data suggest that a gene on HLA-DR4 haplotypes contributes to the level of anti-insulin autoimmunity, and we hypothesize that DR4-associated diabetes susceptibility, distinct from DR3-associated susceptibility, may be secondary to this influence.

Association of polymorphisms in the HLA-B region with extended haplotypes.

Genomic probes from the HLA-B region of the major histocompatibility complex (MHC) were used to study the association of restriction fragment length polymorphisms (RFLPs) with various MHC alleles, complotypes, and extended haplotypes. The two DNA probes, M20A and R5A, were derived from previously cloned cosmids and are located 38 and 110 kilobases (kb) centromeric to HLA-B, respectively. Five different RFLP variants occurring in five different haplotypic combinations were detected within a panel of 40 homozygous-typing cells and cells from 21 families using Bst EII. In two informative families with HLA-B/DR recombinations the inheritance of the RFLP variants was consistent with their mapping between HLA-B and complotypes. The R5A/M20A haplotypic pattern 6.5 kb/3.0 kb (A) had a normal Caucasian frequency of approximately 0.43 and was found in all independent examples of the extended haplotypes [HLA-B8,SC01,DR3], [HLA-B18,F1C30,DR3], [HLA-Bw62,SC33,DR4], [HLA-B44,SC30,DR4], and [HLA-Bw47,FC91,0,DR7]. The patterns 6.9 kb/3.0 kb (B), 6.5 kb/4.7 kb (C), 1.45 kb/3.0 kb (D), and 6.9 kb/4.7 kb (E) had normal Caucasian frequencies of approximately 0.23, 0.15, 0.15, and 0.04 and were found on all independent examples of [HLA-B38,SC21,DR4], [HLA-Bw57,SC61,DR7], [HLA-B7,SC31,DR2], and [HLA-B44,FC31,DR7], respectively. Individual complotypes or HLA-B alleles which were markers of extended haplotypes showed variable associations. For example, HLA-B7 and the complotype SC31 were associated with all R5A/M20A RFLP haplotypes except haplotype E, although [HLA-B7,SC31,DR7] was associated exclusively with haplotype D. HLA-B27, not known to be part of an extended haplotype, was surprisingly exclusively associated with the 6.5 kb/4.7 kb or C haplotypic pattern in all five instances tested. These findings support the concept of regional conservation of DNA on independent examples of extended haplotypes. The results also further characterize these haplotypes.

Complotypes and extended haplotypes in laboratory medicine.

The region on the short arm of the sixth human chromosome encoding class I and class II histocompatibility antigens involved in immune recognition also encodes a group of molecules unrelated to HLA termed class III which includes C2, C4, and factor B. The four genes encoding the complement proteins occupy about 120 kb of genomic DNA between HLA-B and HLA-DR and are closer to HLA-DR. The four genes are inherited as a single unit, without observed crossover, called a complotype, designated by its BF, C2, C4A, and C4B alleles. There are about fifteen complotypes with frequencies of 0.01 or higher on normal caucasian chromosomes. Analysis of linkage disequilibrium between HLA-B, HLA-DR alleles, and complotypes reveals that about 30% of normal caucasian chromosomes consist of fixed sets called extended or fixed haplotypes. There are over a dozen such extended haplotypes defined by their HLA-B, DR and complotype alleles. They appear to contribute most of the previously described linkage disequilibrium between HLA-A/HLA-B and HLA-B/HLA-DR allelic pairs as well as most of the known HLA marker-disease associations. It is postulated that extended haplotypes consist of fixed DNA over at least the 10(6) base pairs of the HLA-B-DR interval, and independent examples in apparently unrelated individuals are thus identical or nearly identical over this interval. A practical consequence of this concept is the possible prediction of successful tissue transplantation donor-recipient pairs.