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.