RESUMEN
Holoprosencephaly of unknown definite causes, has been associated with several chromosome abnormalities involving the autosomes and the sex chromosomes. The most commonly reported associations include dup(3p), del(7q), deletions of chromosome 13, trisomy 13, trisomy 18, and triploidy. In previously reported cases in Korea, none were associated with chromosome 21 anomalies. In conclusion, we reported the first case of holoprosencephaly(semilobar type) associated with pure monosomy 21. We experienced a semilobar type holoprosencephaly with monosomy 21 in a neonate who had multiple congenital anomalies, including an abnormal face, a small thorax with widely spaced hypoplastic nipples and nail hypoplasia, lung hypoplasia with severe scoliosis and cardiac abnormalities. Chromosomal analysis revealed a 45, XY, -21.