NT5C1B (5'-nucleotidase, cytosolic IB) is a cytosolic enzyme that catalyzes 5'-nucleotidase activity and participates in adenosine and purine nucleotide catabolism [GO Annotations]. The gene exhibits cancer-testis antigen (CTA) characteristics, with expression normally restricted to testicular tissue but aberrantly expressed in certain malignancies. NT5C1B mRNA is detected in canine testis and shows expression in canine oral melanoma and metastatic cell lines 1, as well as in canine T-cell lymphomas with low-level expression in some normal dog tissues 2. In humans, NT5C1B has been associated with bulimia nervosa spectrum phenotypes in a genome-wide association study of disordered eating 3, and somatic mutations in NT5C1B have been identified in triple-negative breast cancer patients 4. Despite being reproductive organ-enriched, NT5C1B is dispensable for male fertility, as Nt5c1b gene-deficient mice generated via CRISPR/Cas9 exhibited normal fertility 5. The aberrant CTA expression pattern and association with specific disease phenotypes suggest NT5C1B may serve as an immunotherapy target in certain cancers, though its precise metabolic and clinical roles require further investigation.