The basal cell nevus syndrome (BCNS) is characterized by developmental abnormalities and by the postnatal occurrence of cancers, especially basal cell carcinomas (BCCs), the most common human cancer. Heritable mutations in BCNS patients and a somatic mutation in a sporadic BCC were identified in a human homolog of the Drosophila patched (ptc) gene. The ptc gene encodes a transmembrane protein that in Drosophila acts in opposition to the Hedgehog signaling protein, controlling cell fates, patterning, and growth in numerous tissues. The human PTC gene appears to be crucial for proper embryonic development and for tumor suppression.

Mutations in PATCHED (PTC), the human homolog of the Drosophila patched gene, have been identified in most exons of the gene in patients with the basal cell nevus syndrome and in sporadic basal cell carcinomas. We have screened the 23 PTC exons for mutations using single strand conformation polymorphism analysis of DNA from 86 basal cell nevus syndrome probands, 26 sporadic basal cell carcinomas, and seven basal cell nevus syndrome-associated basal cell carcinomas. This screen identified mutations located in eight exons in 13 of the basal cell nevus syndrome patients and in three of the tumors. The most common mutations were frameshifts resulting in premature chain termination. These results provide further evidence for the crucial role of PTC as a tumor suppressor in human keratinocytes.

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The region surrounding D21S55 in band 21q22 of human chromosome 21 has been implicated in the etiology of Down syndrome (DS). In this paper, we report the construction of a high resolution map of a 2.5 Mb region around the marker D21S55. Characterization of YAC clones by accurate size determination, end isolation and marker assignment was used to build a refined YAC-based map. The YAC clones were then used to isolate 284 cosmid clones, covering 2.3 Mb, from a chromosome 21-specific cosmid library. The cosmid clones were ordered into overlapping groups and a restriction map of each group was determined. The groups of cosmids were then ordered and oriented with respect to the YAC-based map. This high resolution map provides the framework for further analysis of the region by transcribed sequence mapping and sequence determination.