The CCND family includes three mitogen-induced D-type cyclins, CCND1, CCND2 and CCND3, which function as regulatory subunits of the cyclin-dependent kinases CDK4 and CDK6, that drive progression through the G1 phase of the cell cycle. Members in this family contain two cyclin boxes. This model corresponds to the second one. The cyclin box is a protein binding domain.