Human tobacco-related cancers exhibit a high frequency of G to T transversions in the mutation hot spot region of the p53 tumor suppressor gene, possibly the result of specific mutagens in tobacco smoke, most notably benzo [a] An increasing body of molecular epidemiological evidence suggests that regions ofthe p53 gene may be a selective target of environmental carcinogens etiologically associated with specific human cancers (4-19). In this context, a high frequency of G to T transversions in the mutation hot spot regions of the p53 gene (2, 3) have been identified in human hepatocellular carcinomas and strongly correlated with the mutagenic effects of dietary aflatoxin B1 exposurf (4,(13)(14)(15).Similarly, UV-B-specific dipyrimidine G to T transitions in the p53 gene have been characterized in human squamous cell carcinomas of the skin (16,17). Unique mutations in p53 have likewise been reported in radon-associated lung cancers (18) and in human epithelial cells exposed to nickel(II) in vitro (19), providing evidence for an important molecular target for the genotoxic effects of these agents. Of particular clinical and epidemiological interest are the observations of a high frequency of G to T transversions among p53 mutations in tobacco-related human neoplasias, including small-cell and non-small-cell lung cancers (5-8), esophageal carcinomas (10-12), and squamous cell carcinomas of the head and neck (9). In several ofthese studies, the presence of p53 mutations in tumors was strongly correlated with lifetime cigarette consumption (7) or a history of heavy smoking (9).It has been suggested (4-12) that the frequent G to T transversion in the p53 gene in tobacco-related cancers may be associated with specific mutagens in tobacco smoke, most notably activated benzo [a] We have previously investigated the role of p53 alterations in the development and progression of the two-stage mouse skin tumorigenesis model induced by 7,12-dimethylbenz- (24) following hematoxylin/eosin staining. Primary explant cultures of skin tumors were prepared as described (22). Papillomas harvested for analyses were subjected to 35-38 weeks of B[a]P treatment, while carcinomas analyzed were subjected to 50 weeks of carcinogen exposure. In the complete carcinogenesis protocol with DMBA, papillomas and carcinomas used in molecular analyses were obtained from mice subjected to 34-52 weeks of carcinogen exposure. The two-stage carcinogenesis protocol using DMBA as initiator has been described (22