CDKN2D (p19INK4d) is a cyclin-dependent kinase inhibitor that functions primarily as a negative regulator of cell cycle progression. It interacts strongly with CDK4 and CDK6 to inhibit their activity and block G1/S transition 1. Beyond cell cycle control, CDKN2D plays multifaceted roles in cellular senescence, DNA damage response, and apoptosis regulation 2. Mechanistically, CDKN2D expression increases during cellular stress and is upregulated in senescent cells through both p53-dependent and p53-independent pathways 3. The gene is transcriptionally repressed by the VDR-Ezh2 axis, which maintains H3K27me3 enrichment at the CDKN2D locus 4. Clinically, CDKN2D elevation associates with age-related diseases. In Alzheimer's disease, CDKN2D-expressing neurons with tau neurofibrillary tangles exhibit senescence hallmarks including enlarged nuclei and lipofuscin accumulation 5. In renal ischemia, upregulated p19 (along with p16 and p21) contributes to maladaptive tubular cell senescence and chr19 kidney dysfunction 6. Additionally, CDKN2D is a component of a validated 5-gene peripheral blood signature predicting immunotherapy response in non-small cell lung cancer 7, suggesting potential utility as a biomarker for treatment stratification.