The Rab27 GTPase subfamily consists of two closely related homologs, Rab27a and Rab27b. Northern blot analysis revealed that Rab27b is expressed significantly only in testis, in which the predominant transcript was 1.4 kb in size. An 8-kb mRNA of unknown origin was detected in many tissues. Rab27b is associated with tubulovesicle membranes in the parietal cell and Rab27b may play a role in stimulation-associated membrane recruitment and gastric acid secretion. Rab27b is recently reported as a marker for breast cancer progression. It regulates invasive growth and metastasis in ER-positive breast cancer cell lines, and increased expression is associated with poor prognosis in humans.