Cyclin D3 (34kDa) is one of three D-type cyclins that are synthesized in G1 phase and induced in response to agents that promote re-entry into the cell cycle. D-type cyclins assemble with cyclin-dependent kinases Cdk4 and Cdk6 to form complexes that phosphorylate key substrates, including retinoblastoma gene product (pRB), involved in proliferation. Expression of cyclin D3 occurs late in the G1 phase of the cell cycle. Unlike cyclins D2 and D1, cyclin D3 is not induced by CSF-1 stimulation in macrophages. Cyclin D3 has been shown to associate with PCNA (Proliferating Cell Nuclear Antigen) and p21, suggesting a possible role for cyclin D3 in DNA replication and repair.