The p27/Kip1 protein belongs to the recently identified family of proteins called cyclin-dependent kinase inhibitors. These proteins play an important role as negative regulators of cell cycle-dependent kinase activity during progression of the cell cycle. Since cyclin-dependent kinase inhibitors can inhibit cell proliferation, they may have a role as tumor suppressor genes. To determine whether p27 alterations may be involved in tumorigenesis, we examined its mutational status in 36 primary breast carcinomas and 9 breast cancer cell lines using PCR-single-strand conformational polymorphism, direct DNA sequencing, and Southern blot analysis. Southern blot analysis showed no homozygous deletions of the p27 gene in either the clinical samples or cell lines. Two point mutations were found in primary tumors. One represents a previously undescribed polymorphism at codon 142; another is a nonsense mutation at codon 104. The latter mutation was absent in the normal matched control sample, and, in addition, it was accompanied with the loss of heterozygosity (LOH) of a microsatellite marker in the vicinity of the p27 gene on chromosome 12p13. These data indicate that p27 mutations are a rare event in breast cancer, but may play an important role in the development of a minority of these cancers. Furthermore, LOH analysis of the 12p13 locus revealed that an additional four of six matched DNA samples had LOH at 12p13 but did not have an alteration of the p27 gene, suggesting that another tumor suppressor gene is located on the short arm of human chromosome 12 which may be frequently involved in the pathogenesis of breast cancers.