The aim of the present study is to address whether the molecular pathogenesis is identical among multiple basal cell carcinomas (BCCs) present in the same nevoid basal cell carcinoma syndrome (NBCCS) patient. Patient 1 is a 61-year-old (yo) Japanese female whose clinical characteristics and findings of a genetic analysis of PTCH1 have been previously described. Patient 2 is patient 1's 64-yo sister who also suffered from NBCCS with a single base deletion at nucleotide 2613 in exon 16 (c.2613delC) in one PTCH1 allele. Thirteen and 3 independent specimens of BCC were applied for a molecular analysis of loss of heterozygosity (LOH) in PTCH1 in patients 1 and 2, respectively. Of particular note is that all BCC specimens examined showed a loss of the wild-type allele of exon 16 in PTCH1, thus indicating that LOH results in the biallelic disruption of PTCH1 in multiple BCCs that develop in an age- and location-independent manner in the same patient. These results indicate that the germline single base deletion of PTCH1 (c.2613 delC) is a first hit and the LOH of the wild-type allele is a second hit, implying that all 16 BCCs detected in these NBCCS sisters fit the standard two-hit model.