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 (PubMed:1833066, PubMed:1827756, PubMed:8114739, PubMed:8302605, PubMed:19412162, PubMed:33854235). 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 (PubMed:1833066, PubMed:1827756, PubMed:8114739, PubMed:8302605, PubMed:19412162). Hypophosphorylates RB1 in early G(1) phase (PubMed:1833066, PubMed:1827756, PubMed:8114739, PubMed:8302605, PubMed:19412162). Cyclin D-CDK4 complexes are major integrators of various mitogenenic and antimitogenic signals (PubMed:1833066, PubMed:1827756, PubMed:8302605, PubMed:19412162). Also a substrate for SMAD3, phosphorylating SMAD3 in a cell-cycle-dependent manner and repressing its transcriptional activity (PubMed:15241418). Component of the ternary complex, cyclin D1/CDK4/CDKN1B, required for nuclear translocation and activity of the cyclin D-CDK4 complex (PubMed:9106657). Exhibits transcriptional corepressor activity with INSM1 on the NEUROD1 and INS promoters in a cell cycle-independent manner (PubMed:16569215, PubMed:18417529). Interacts with either CDK4 or CDK6 protein kinase to form a serine/threonine kinase holoenzyme complex (PubMed:8114739, PubMed:19237565). The cyclin subunit imparts substrate specificity to the complex (PubMed:20399237, PubMed:19237565, PubMed:8302605, PubMed:9106657). Component of the ternary complex CCND1/CDK4/CDKN1B required for nuclear translocation and modulation of CDK4-mediated kinase activity (PubMed:9106657). Interacts directly with CDKN1B (By similarity). Can form similar complexes with either CDKN1A or CDKN2A (By similarity). Interacts with FBXO4 (By similarity). Interacts with UHRF2; the interaction ubiquitinates CCND1 and appears to occur independently of phosphorylation (PubMed:21952639). Interacts with USP2 (PubMed:19917254). 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 (PubMed:16569215, PubMed:18417529, PubMed:19124461). Interacts with CDK4; the interaction is prevented with the binding of CCND1 to INSM1 during cell cycle progression (PubMed:19124461). Cyclin D-CDK4 complexes accumulate at the nuclear membrane and are then translocated to 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 (PubMed:10766840, PubMed:33854232, PubMed:33854235, PubMed:33854239). It also plays an essential role for recognition by the FBXO31 component of SCF (SKP1-cullin-F-box) protein ligase complex following DNA damage (PubMed:19412162). 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 (PubMed:33854232, PubMed:33854235, PubMed:33854239). The DCX(AMBRA1) complex represents the major regulator of CCND1 stability during the G1/S transition (PubMed:33854232, PubMed:33854235, PubMed:33854239). 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 (PubMed:19412162). SCF-type ubiquitination is dependent on Thr-286 phosphorylation (PubMed:10766840, PubMed:19412162). Ubiquitinated also by UHRF2 apparently in a phosphorylation-independent manner (PubMed:21952639). Ubiquitination leads to its degradation and G1 arrest. Deubiquitinated by USP2; leading to its stabilization (PubMed:19917254). A chromosomal aberration involving CCND1 may be a cause of B-lymphocytic malignancy, particularly mantle-cell lymphoma (MCL). Translocation t(11;14)(q13;q32) with immunoglobulin gene regions. Activation of CCND1 may be oncogenic by directly altering progression through the cell cycle. A chromosomal aberration involving CCND1 may be a cause of parathyroid adenomas. Translocation t(11;11)(q13;p15) with the parathyroid hormone (PTH) enhancer. The gene represented in this entry is involved in disease pathogenesis. A chromosomal aberration involving CCND1 is found in multiple myeloma. Translocation t(11;14)(q13;q32) with the IgH locus. Belongs to the cyclin family. Cyclin D subfamily.