Using gene mapping, combined with the known genetic information collected from the linkage studies, it was possible to identify germ-line loss-of-function variants in TSC1 (chromosome 9q34; hg38: g.9:132891348-132946874; NM_000368.5; 23 exons) and TSC2 (chromosome 16p13.3; hg38: g.16:2047985-2089491; NM_000548.5; 42 exons) as the primary cause of TSC (van Slegtenhorst et al., 1997; European Chromosome 16 Tuberous Sclerosis Consortium, 1993). The gene discussed is TSC1; the disease is tuberous sclerosis.