The INK4a locus encodes two different, cell cycle-regulating proteins, p16INK4a and p19ARF. In this study, we screened mutations in all coding regions of the INK4a locus (exons 1 beta, 1 alpha, 2, and 3) in 21 squamous cell carcinomas (SCCs) of human skin by polymerase chain reaction-single strand conformation polymorphism analysis. Mutations were detected in 3 SCCs in exon 2, which is common to both p16INK4a and p19ARF. These included an in-frame deletion of 21 base pairs from codon 84 to 90, a frameshift mutation of CCC-->TC at codon 75, and a nonsense mutation of CGA-->TGA at codon 80 of the p16INK4a gene. These results suggest that inactivation of the INK4a locus has some relevance to the carcinogenesis in at least some of SCCs of human skin. This is the first demonstration of aberrations in the INK4a locus in SCCs of human skin.