NT5C1A encodes a cytosolic 5'-nucleotidase that catalyzes the hydrolysis of ribonucleotide and deoxyribonucleotide monophosphates to release inorganic phosphate and the corresponding nucleoside 1. AMP is the major substrate, though the enzyme also hydrolyzes dCMP and IMP 1. The enzyme is expressed at high levels in skeletal and heart muscle, with intermediate expression in pancreas and brain 1. NT5C1A plays a role in regulating physiological nucleotide pools and can influence nucleoside analog metabolism; in cell culture models, its expression conferred resistance to multiple chemotherapeutic nucleoside analogs including 2-chloro-2'-deoxyadenosine and 2',3'-difluorodeoxycytidine 1. Clinically, NT5C1A has emerged as an important autoimmune target. Circulating autoantibodies against NT5C1A are detected in approximately 61% of inclusion body myositis (IBM) patients but occur less frequently in polymyositis, dermatomyositis, and other systemic autoimmune diseases 2. Anti-NT5C1A antibodies represent the only known serum biomarker specific to IBM, though they also appear in other idiopathic inflammatory myopathies and autoimmune conditions 3. The antibodies show moderate sensitivity and high specificity for IBM diagnosis 4. Among IBM patients, anti-NT5C1A seropositivity correlates with increased cytochrome oxidase-negative fibers 5, though antibody status does not reliably predict treatment response or survival outcomes 5.