Regulatory component of the cyclin D2-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. Phosphorylation of RB1 allows dissociation of the transcription factor E2F from the RB/E2F complex and the subsequent transcription of E2F target genes which are responsible for the progression through the G(1) phase. Hypophosphorylates RB1 in early G(1) phase. Cyclin D-CDK4 complexes are major integrators of various mitogenenic and antimitogenic signals. Interacts with either CDK4 or CDK6 protein kinase to form a serine/threonine kinase holoenzyme complex. The cyclin subunit imparts substrate specificity to the complex. Cyclin D-CDK4 complexes accumulate at the nuclear membrane and are then translocated into the nucleus through interaction with KIP/CIP family members. Phosphorylation at Thr-279 by MAP kinases is required for ubiquitination and degradation by the DCX(AMBRA1) complex. Ubiquitinated by the DCX(AMBRA1) complex during the transition from G1 to S cell phase, leading to its degradation: ubiquitination is dependent on Thr-279 phosphorylation. The DCX(AMBRA1) complex represents the major regulator of CCND2 stability during the G1/S transition (By similarity). Polyubiquitinated by the SCF(FBXL2) complex, leading to proteasomal degradation (By similarity). Belongs to the cyclin family. Cyclin D subfamily.