We have investigated a family in which three siblings with the autosomal 
dominant disorder tuberous sclerosis had unaffected parents. The family were 
typed for polymorphic markers spanning the two genes known to cause tuberous 
sclerosis located at 9q34 (TSC1) and 16p13.3 (TSC2). TSC1 markers showed 
different maternal and paternal haplotypes in affected children, excluding a 
mutation in TSC1 as the cause of the disease. For the TSC2 markers all the 
affected children had the same maternal and paternal haplotypes, as did three of 
their unaffected siblings. Mutation screening by RT-PCR and direct sequencing of 
the TSC2 gene identified a 4 bp insertion TACT following nucleotide 2077 in exon 
18 which was present in the three affected children but not in five unaffected 
siblings or the parents. This mutation would cause a frameshift and premature 
termination at codon 703. Absence of the mutation in lymphocyte DNA from the 
parents was consistent with germline mosaicism and this was confirmed by our 
finding of identical chromosome 16 haplotypes in affected and unaffected 
siblings, providing unequivocal evidence of two different cell lines in the 
gametes. Molecular analysis of the TSC2 alleles present in the affected subjects 
showed that the mutation had been inherited from the mother. This is the first 
case of germline mosaicism in tuberous sclerosis proven by molecular genetic 
analysis and also the first example of female germline mosaicism for a 
characterized autosomal dominant gene mutation apparently not associated with 
somatic mosaicism.