GTPase-activating protein specific for Rab1 and Rab2 small GTPase families for which it can accelerate the intrinsic GTP hydrolysis rate by more than five orders of magnitude. The arginine and glutamine fingers are critical for the GTPase-activating mechanism, they pull out Rab's 'switch 2' glutamine and insert in Rab's active site. Defects in Tbc1d20 are the cause of spontaneous autosomal recessive blind sterile (bs) phenotype. Bs animals exhibit embryonic non-progressive nuclear cataracts and spermatid abnormalities associated with male infertility.