CDK5R1 encodes p35, a neuron-specific activator of cyclin-dependent kinase 5 (CDK5) essential for central nervous system development. p35/CDK5 complexes regulate neurite outgrowth, cortical lamination, and dendritic spine morphogenesis through EFNA1-EPHA4 signaling 1. The complex also modulates circadian clock function by phosphorylating CLOCK protein and regulating CLOCK-BMAL1 transcriptional activity. CDK5R1 expression is tightly controlled at transcriptional and post-transcriptional levels, with microRNAs miR-103 and miR-107 directly targeting the CDK5R1 3'-UTR to regulate p35 levels and neuronal migration 2. In disease contexts, CDK5R1 dysregulation associates with Alzheimer's disease pathology; epistatic interactions between CDK5R1 and GSK-3beta polymorphisms influence AD susceptibility through tau phosphorylation regulation 3. CDK5R1 downregulation characterizes AD pathogenesis with identified disease subtypes 4, while tau-related neurodegeneration involves altered CDK5R1 expression and lipid dyshomeostasis 5. Conversely, CDK5R1 upregulation promotes oncogenesis; elevated expression correlates with poor prognosis and chemotherapy resistance in colorectal, breast, and hepatocellular cancers 67, with the miR-152-CDK5R1 axis driving Ewing's sarcoma proliferation through CDK5-mediated retinoblastoma phosphorylation 8.