G protein-coupled receptor 15 (GPR15) is an Orphan-A GPCR that binds C10orf99 and thrombomodulin. It is a coreceptor for HIV and may play a role in HIV-1 pathogenesis and transmission. Expression of this receptor can be elevated on regulatory T cells in peripheral blood in inflammatory bowel diseases, and it plays a role in T cell homing to inflamed regions of the gut. In immunohistochemistry of normal tissues, GPR15 has high membranous positivity in surface epithelium of the colon and small bowel, hepatocytes in the liver and spleen, moderate positivity in the thymus, lymph nodes, kidney tubular epithelium, prostate, testes, glandular cells of the stomach, and it is found on T cells in blood. In cancer, it is expressed in glioblastoma, melanoma, colorectal carcinoma and small cell lung carcinoma. References: Gut. 2016 Oct;65(10):1642-64, PMID: 26209553; Am J Pathol. 2001. 159:1933-9, PMCID: PMC1867054;