AHCYL1 is a multifunctional cellular regulator coordinating epithelial secretion, calcium homeostasis, and apoptosis. It acts as an endogenous inhibitor of the inositol 1,4,5-trisphosphate receptor (ITPR1), competing for ligand binding and suppressing calcium release under basal conditions. When phosphorylated, AHCYL1 dissociates from ITPR1 to coordinate activation of CFTR, SLC26A6, and SLC4A4, thereby stimulating electrolyte and bicarbonate secretion in pancreatic and salivary ducts. Under apoptotic stress, dephosphorylation of AHCYL1 promotes ER-to-mitochondrial calcium transfer, facilitating programmed cell death. AHCYL1 also regulates mRNA polyadenylation and controls ribonucleotide reductase activity to modulate dNTP pools during cell cycle progression. In cancer, AHCYL1 exhibits context-dependent roles. In NRAS-mutated melanoma, AHCYL1 is upregulated and required for tumor growth through calcium-dependent mechanisms; AHCYL1 inhibition triggers unfolded protein response and apoptosis 1. By contrast, AHCYL1 acts as a tumor suppressor in non-small cell lung cancer, where low expression correlates with stemness and enhanced tumorigenicity 2. In non-small cell lung carcinoma, AHCYL1 enhances PREX2 GEF activity to promote tumor growth 3. AHCYL1 also supports hepatitis C virus replication by interacting with viral nonstructural 5A protein 4. Cardiac overexpression of AHCYL1 disrupts calcium signaling and induces hypertrophic remodeling with impaired function. These findings suggest AHCYL1 as a therapeutic target in NRAS-mutated cancers and select carcinomas.