WE have previously identified a candidate oncogene (PRAD1 or D11S287E) on chromosome Ilql3 which is clonally rearranged with the parathyroid hormone locus in a subset of benign parathyroid tumours^1,2. We now report that a cloned human placental PRAD1 complementary DNA encodes a protein of 295 amino acids with sequence similarities to the cyclins. Cyclins can form a complex with and activate p34^cdc2 protein kinase, thereby regulat-ing progress through the cell cycle (reviewed in refs 3–5). PRAD1 messenger RNA levels vary dramatically across the cell cycle in HeLa cells. Addition of the PRAD1 protein to interphase clam embryo lysates containing inactive p34cdc2 kinase and lacking endogenous cyclins allows it to be isolated using beads bearing pl3^suc1, a yeast protein that binds cdc2 and related kinases with high affinity and coprecipitates kinase-associated proteins. Addition of PRAD1 also induces phosphorylation of histone HI, a preferred substrate of cdc2. These data suggest that PRAD1 encodes a novel cyclin whose overexpression may play an important part in the development of various tumours with abnormalities in 11q13.