PRAD1 (cyclin D1) has been implicated in the molecular pathogenesis of a variety of tumors, including parathyroid adenomas, t(11;14)-bearing B-lymphoid tumors, and breast cancer. The sequence of the overexpressed PRAD1 genes has been directly analyzed in only two tumor specimens, a benign parathyroid adenoma and a malignant centrocytic lymphoma. Thus, little is known about PRAD1 sequence in the vast majority of human primary tumors, including breast cancers. Using single-strand conformational polymorphism (SSCP) analysis, we have examined the coding region of the PRAD1 gene in 30 primary breast cancers and 25 parathyroid adenomas. Polymerase chain reaction (PCR)-SSCP analysis of the coding region of exons 1-5 of the PRAD1 gene did not reveal any tumor-specific mutations. During the course of screening for mutations, we found and established the sequence variants of a new DNA polymorphism at codon 241 within exon 4 of the PRAD1 gene. Since this polymorphism is located within the coding region of the PRAD1 gene, it will allow determination of allele-specific expression of the gene and the detection of allele imbalance. At least in breast and parathyroid neoplasms, overexpression of the wild-type PRAD1 sequence, rather than point mutational activation, appears to be the predominant mechanism by which PRAD1 exerts its oncogenic action.