CDK5R2 (p39) is a regulatory subunit that activates cyclin-dependent kinase 5 (CDK5), an atypical serine/threonine kinase 1. As a non-cyclin activator protein, CDK5R2 enables CDK5's diverse cellular functions beyond its canonical neuronal roles 1. In the nervous system, CDK5R2-associated Cdk5 activity critically regulates dendritic morphogenesis by modulating cytoskeletal organization, protein synthesis, and trafficking through PI3K/Akt, cAMP, and small GTPase signaling pathways 2. Neurons deficient in p39 exhibit reduced dendritic arborization comparable to CDK5-deficient neurons 2. Beyond neurodevelopment, CDK5R2 participates in neuroendocrine differentiation in castration-resistant prostate cancer, where ENZ treatment upregulates CDK5R1/R2 expression leading to RB1 phosphorylation and neuroendocrine gene expression 3. CDK5R2 emerges as a disease-relevant biomarker in multiple pathologies: it is a hub gene in HIV encephalitis 4, part of regulatory networks in medullary thyroid carcinoma immune characteristics 5, identified in hepatocellular carcinoma prognostic models 6, and implicated in HIV-1 replication control 7. Additionally, CDK5R2 was identified as a vulnerable kinase target in multiple myeloma 8, suggesting therapeutic potential for kinase inhibition strategies across diverse malignancies.