second cyclin box found in G1/S-specific cyclin-D3 (CCND3)
CCND3 is a regulatory component of the cyclin D3-CDK4 (DC) complex that phosphorylates and inhibits members of the retinoblastoma (RB) protein family, including RB1, and regulates the cell-cycle during G(1)/S transition. In skeletal muscle, CCND3 plays a unique function in controlling the proliferation/differentiation balance of myogenic progenitor cells. CCND3 contains two cyclin boxes. This model corresponds to the second one. The cyclin box is a protein binding domain.