GRTP1 (Growth hormone-regulated TBC protein 1), also designated TBC1 domain family member 6, is a 336 amino acid protein with expression modulated by growth hormone. The TBC1 domain of GRTP1 is a region that is notably conserved among GTPase-activating proteins, which characteristically play a role in the cell cycle and mitosis coordination. Interestingly, expression of GRTP1 is significantly upregulated in post-puberty testis and then maintained in the adult, suggesting that the germ cell may be the major site of GRTP1 function. There are two isoforms of GRTP1, which are a result of alternative splicing events. May act as a GTPase-activating protein for Rab family protein(s).