Six families with mosaicism in a series of 62 unrelated families with a mutation in one of the two tuberous sclerosis complex (TSC) genes, TSC1 or TSC2, are reported from the Erasmus University and Hospital, Rotterdam, The Netherlands. In one family the parents showed no clinical signs, but gonadal mosaicism was determined after the diagnosis of TSC was made in 3 children. The exclusion of signs of TSC in the parents reduces the likelihood of a mosaic mutation parental carrier from 10% to 2%. In 5 families with somatic mosaicism, TSC was diagnosed in the parent after the child’s case was identified. The genetic counseling implications of mosaicism are discussed. 
COMMENT. The authors advise complete clinical screening (including brain CT scan) of parents of sporadic TSC patients. A mild phenotype associated with somatic mosaicism might otherwise be missed. In the absence of signs of TSC, only 2% of parents will show gonadal mosaicism.