The prevalence and CT appearance of the levator claviculae muscle: a normal variant not to be mistaken for an abnormality.

BACKGROUND AND PURPOSE: The levator claviculae muscle is an infrequently recognized variant in humans, occurring in 2% to 3% of the population, and has rarely been reported in the radiologic or anatomic literature. The importance of this muscle to radiologists is in distinguishing it from an abnormality; most commonly, cervical adenopathy. After discovering this muscle on the CT scans of two patients during routine clinical examinations, we conducted a study to determine the prevalence and appearance of the muscle on CT studies. METHODS: We evaluated 300 CT scans that adequately depicted the expected location of the muscle. The most superior level in which the muscle could be identified and the apparent location of insertion on the clavicle were recorded for all subjects in whom the muscle was detected. RESULTS: Seven levator claviculae muscles were identified in six subjects (2%). It was bilateral in one, on the left in four, and on the right in one. It was identified up to the level of the transverse process of C3 in all cases. The insertion was the middle third of the clavicle for two muscles and the lateral third of the clavicle for the remaining five muscles. CONCLUSION: Because the levator claviculae muscle will most likely be encountered during a radiologist's career, it is important to recognize this muscle as a variant and not as an abnormality.

Telomerase RNAs of different ciliates have a common secondary structure and a permuted template.

Telomerase is composed of protein and RNA. The RNA serves as a template for telomere DNA synthesis and may also be important for enzyme structure or catalysis. We have used the presence of conserved sequence elements in the promoter and template regions to amplify by PCR the telomerase RNA genes from six different hypotrichous ciliates: Oxytricha nova, Oxytricha trifallax, Stylonychia mytilis, Stylonychia lemnae, Euplotes aediculatus, and Euplotes eurystomus. RNaseH cleavage of the O. nova RNA in extracts by use of a complementary oligonucleotide leads to loss of telomerase activity, supporting the identification. Primary sequence and biochemical experiments suggest that the templates of Oxytricha and Stylonychia are circularly permuted relative to that of E. aediculatus. On the basis of the pause sites, the former two add G4T4 during a single primer elongation cycle, whereas E. aediculatus adds G3T4G. The only primary sequence element outside the template that is conserved between these phylogenetically distant telomerase RNAs is the sequence 5'-(C)UGUCA-3', which precedes the template regions by exactly two bases. We propose a common secondary structure model that is based on nucleotide covariations, a model which resembles that proposed previously for tetrahymenine telomerase RNAs.