The PRAD1/cyclin D1 gene (CCND1), a member of the D-type cyclin gene family, has been implicated as a protooncogene in parathyroid, lymphoid, and mammary tumors. We cloned and mapped another member of this family, the human cyclin D3 gene (CCND3), to chromosome 6p→q13 using human × rodent hybrids. This assignment raises the hypothesis that cyclin D3 may be involved in the pathogenesis of human neoplasms with abnormalities of chromosome 6.