Abstract

A patient is reported with mental retardation, facial and body asymmetry, and hyperpigmented areas limited to the right side of the body. Cytogenetic studies revealed trisomy 18 in 50% of peripheral blood lymphocytes; fibroblast cultures from the hyperpigmented area showed pure trisomy 18, while the karyotype on the unaffected side was normal. This could be an example of the 'lines of Blaschko', considered to be a form of 'human mosaicism', in which an abnormality occurred in melanocytes migrating from the neural crest. Non-disjunction of one chromosome 18 appears to be associated with the mutational event that caused abnormal migration of melanoblasts.