The p57KIP2 gene encodes an inhibitor of cyclin-dependent kinase activity, which negatively regulates cell cycle progression. The human p57 gene is located in 11p15.5, a region of DNA frequently altered in neoplasia. We have isolated a human genomic clone and mapped the p57 gene to a 2.2-kb region between D11S648 and D11S679. Sequence analysis revealed that the coding DNA of the human p57 gene is divided by 0.5-kb intron. A second intron was detected in the 3' untranslated region, indicating that the human p57 gene contains at least three exons. Our previous work with somatic cell hybrids mapped a tumor suppressor gene for the G401 Wilms' tumor cell line to a approximately 500-kb region of 11p15.5 that includes p57. Northern blot analysis detected a 0.8-kb p57 transcript in several of the G401 hybrid lines. However, p57 expression did not correlate with tumor suppression. These results suggest that p57 is not responsible for the tumor suppression observed in our somatic cell hybrid assay.