NUMB is a multifunctional endocytic adaptor protein that regulates clathrin-mediated receptor endocytosis 1 and serves as a critical tumor suppressor across multiple cancer types. In normal physiology, NUMB functions as a cell density sensor that couples external cell-cell contact signals to the Hippo-YAP pathway through interaction with phosphorylated SPTAN1, thereby restraining cell growth and preventing loss of contact inhibition 2. NUMB also governs cell fate decisions through mitochondrial quality control via Parkin-mediated mitophagy; deficiency in this pathway promotes metabolic reprogramming that drives neuroendocrine differentiation and therapeutic resistance in adenocarcinomas 3. Additionally, NUMB regulates intracellular trafficking of the uric acid transporter ABCG2 to the apical plasma membrane in renal tubular epithelial cells, and NUMB dysfunction causes hereditary hyperuricemia and gout 4. In cancer pathogenesis, NUMB loss promotes aggressive bladder cancer through RHOA/ROCK-dependent YAP activation 5 and regulates Notch1 receptor degradation; CD51-mediated Numb inhibition promotes gastric cancer stemness by blocking Notch1 degradation 6. NUMB downregulation in liver cancer involves selective upregulation of non-functional NUMB isoforms lacking phosphotyrosine-binding domains 2. These findings identify NUMB loss as a biomarker for high-risk patients potentially benefiting from targeted therapies.