Regulatory component of the cyclin D1-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. Also a substrate for SMAD3, phosphorylating SMAD3 in a cell-cycle-dependent manner and repressing its transcriptional activity. Component of the ternary complex, cyclin D1/CDK4/CDKN1B, required for nuclear translocation and activity of the cyclin D-CDK4 complex. Exhibits transcriptional corepressor activity with INSM1 on the NEUROD1 and INS promoters in a cell cycle-independent manner. 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. Component of the ternary complex CCND1/CDK4/CDKN1B required for nuclear translocation and modulation of CDK4-mediated kinase activity (By similarity). Interacts directly with CDKN1B. Can form similar complexes with either CDKN1A or CDKN2A. Interacts with FBXO4 (By similarity). Interacts with UHRF2; the interaction ubiquitinates CCND1 and appears to occur independently of phosphorylation. Interacts with USP2. Interacts (via cyclin N-terminal domain) with INSM1 (via N-terminal region); the interaction competes with the binding of CCND1 to CDK4 during cell cycle progression and inhibits CDK4 activity. Interacts with CDK4; the interaction is prevented with the binding of CCND1 to INSM1 during cell cycle progression (By similarity). 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-286 by MAP kinases is required for ubiquitination and degradation by the DCX(AMBRA1) complex. It also plays an essential role for recognition by the FBXO31 component of SCF (SKP1-cullin-F-box) protein ligase complex following DNA damage. Ubiquitinated at Lys-269 by the DCX(AMBRA1) complex during the transition from G1 to S cell phase, leading to its degradation: ubiquitination is dependent on Thr-286 phosphorylation. The DCX(AMBRA1) complex represents the major regulator of CCND1 stability during the G1/S transition (By similarity). Also ubiquitinated by a SCF (SKP1-CUL1-F-box protein) ubiquitin-protein ligase complex containing FBXO4 and CRYAB (By similarity). Following DNA damage it is ubiquitinated by some SCF (SKP1-cullin-F-box) protein ligase complex containing FBXO31. SCF-type ubiquitination is dependent on Thr-286 phosphorylation. Ubiquitinated also by UHRF2 apparently in a phosphorylation-independent manner. Ubiquitination leads to its degradation and G1 arrest. Deubiquitinated by USP2; leading to its stabilization (By similarity). Belongs to the cyclin family. Cyclin D subfamily.