Cyclins regulate cell cycle progression by complexing with and activating cdc2 or related kinases. PRAD1/cyclin D1 is a recently discovered putative oncogene in several types of human tumors and may regulate G1-S phase progression. We have cloned a related human cDNA, called cyclin D3, from a placental cDNA library by cross-hybridization with PRAD1. In synchronized HeLa cells, the mRNA levels of PRAD1 and cyclin D3 were regulated reciprocally through the cell cycle: cyclin D3 mRNA levels peaked in S phase, where PRAD1 mRNA was lowest in S. In normal human mammary epithelial (70N) cells synchronized by growth factor deprivation and subsequent growth factor stimulation, PRAD1 expression peaked in G1 and declined before S phase, while cyclin D3 expression rose later in G1 and remained elevated in S. Therefore, the close relationship (53.1% identity) between PRAD1 and cyclin D3 does not necessarily imply redundant functions of these candidate G1 cyclins; they may have distinct roles in progression from G1 through S phase.