CCND3 encodes cyclin D3, a regulatory protein that forms complexes with CDK4 and CDK6 to control cell cycle progression through the G1/S transition 1. The cyclin D3-CDK4/6 complexes phosphorylate retinoblastoma (RB) family proteins, leading to E2F transcription factor release and subsequent S-phase gene expression 1. CCND3 acts as a critical integrator of mitogenic and antimitogenic signals, coupling extracellular growth factors to cell cycle machinery 1. Beyond cell cycle regulation, CCND3 demonstrates antiviral properties, particularly against bandaviruses like SFTSV, where it inhibits viral replication by disrupting nucleoprotein multimerization and RNA binding 2. Dysregulation of CCND3 contributes to multiple malignancies through different mechanisms. In multiple myeloma, chr6 translocations at 6p21 dysregulate CCND3 expression 3. In Burkitt lymphoma, oncogenic CCND3 mutations create highly stable cyclin D3 isoforms that drive aberrant cell cycle progression 4. Paradoxically, CCND3 downregulation in cisplatin-resistant lung adenocarcinoma promotes metastasis by reducing PARK2-mediated vimentin degradation, triggering epithelial-mesenchymal transition 5. CCND3 mutations also occur in pathogenic autoantibody-producing B cells, representing a pre-neoplastic stage in lymphomagenesis 6.