Wolf-Hirschhorn syndrome is caused by a partial deletion of chromosome 4. The most commmon abnormalties seen are severe mental retardation, microcephaly, siezures, hypotonia, and cleft lip and/or palate. Characteristic facial features, include stabismus, hypertelorism, down-turned "fishlike" mouth, short upper lip and philtrum, small chin, ear tags or pits, and cranial asymmetry. About 87% of cases represent a de novo deletion, while about 13% are inherited from a parent with a chromosome translocation. The critical region for determining the phenotype is at 4p16.3.