AbstractRecent evidence from molecular biology studies of the cell cycle machinery suggests that, apart from oncogenes and tumor suppressor genes, the genes encoding the key cell cycle regulatory proteins could serve as additional targets for oncogenic mutations involved in the multistep process of car‐cinogenesis. In an attempt to identify such potential cancer‐associated aberrations of the cell cycle regulators, the expression of cdc2 and cdk2 kinases, as well as cyclins A, Bl and Dl, was analyzed by immunoblotting in a panel of more than 40 human cancer cell lines derived from 17 different tumor types. The expression of cdc2, cdk2, cyclin Bl and cyclin A polypeptides was detectable in all lines examined, and moderate variation in protein level does not provide evidence for any obvious abnormalities in the cancer cell lines studied. The application of a series of novel monoclonal antibodies (Mab) to human cdc2 revealed the existence of an intriguing protein, designated p37, immunologically and structurally related to cdc2, which is strongly and selectively expressed in about 50% of the cancer cell lines. In contrast to cyclin A, which has also been implicated in tumorigenesis, we found pronounced variation in abundance of the cyclin Dl protein. Our data suggest that dysregulation of cyclin Dl (a candidatebcl‐1, PRAD1 oncogene) can be involved in the path‐ogenesis of some additional tumor types (e.g., sarcomas and neuroblastomas) besides those reported for amplification and/or mRNA overexpression of this oncogene. The cancer cell lines with high and extremely low levels of cyclin Dl identified in the present study could represent suitable in vitro models to further elucidate the role of this cyclin in the process of neoplastic transfo