RAB27A and cancer: As noted above, Rab27A is emerging as a key regulator of metastatic behaviour in various cancers,8–12 and Rab27A inhibition should carry a low risk of mechanism-based toxicity in adult tissues.14 Like many other GTPases, Rab27A is a highly challenging target for small molecule inhibitors; however, the SF4 pocket is required for selective Rab27A-effector interactions and may afford a route to target this key PPI and disrupt function.