GRTP1 (growth hormone regulated TBC protein 1) is a 258 amino acid protein that functions as a GTPase-activating protein for Rab-like small GTPases, containing the characteristic TBC signature motif 1. The gene is primarily expressed in testicular tissue, with post-pubertal upregulation in testes and lower expression in intestine, kidney, lung, and liver 1. GRTP1 expression is growth hormone-responsive, increasing significantly in testes (140%) following GH administration 1. In testicular function, GRTP1 plays an important role, as evidenced by reduced expression in Rex1-deficient mice that exhibit impaired spermatogenesis, abnormal sperm morphology, and testicular atrophy 2. Beyond reproductive function, GRTP1 expression is dysregulated in pathological conditions; peripheral blood GRTP1-AS1 (antisense transcript) levels decrease following integrated traditional Chinese medicine treatment in sciatica patients, correlating with pain relief 3. GRTP1 is also upregulated in mouse B cells in response to IL-4 stimulation 4, suggesting involvement in immune cell activation. The conservation of GRTP1 orthologs across human, mouse, rat, and Drosophila indicates evolutionary importance 1. These findings suggest GRTP1 regulates Rab GTPase signaling in reproductive and potentially immune contexts.