Somatic and germ line mosaicism and mutation origin for a mutation in the L1 gene in a family with X-linked hydrocephalus

X-linked hydrocephalus is caused by mutations in the gene for neural cell adhesion molecule L1 (L1CAM). In this report, we describe identification of a mutation in an isolated case of hydrocephalus with adducted thumbs. Tracing the origin of the mutation within the family showed a degree of somatic mosaicism in the asymptomatic maternal grandfather of the propositus. This report highlights the need to take mosaicism into account when counselling relatives of affected individuals. Am. J. Med. Genet. 75:200–202, 1998. © 1998 Wiley-Liss, Inc.