RAB31, also referred to as RAB22B, is a member of the RAB5 family of proteins (Chen et al, 1996; Bao et al, 2002). It is involved in trafficking pathways at the trans-Golgi network (TGN) and early endosome, where it is required for insulin-stimulated GLUT4 transport, internalization of the EGFR receptor and for mannose-6-phosphate receptor transport from the TGN (Rodriguez-Gabin et al, 2001; Lodhi et al, 2007; Chua et al, 2014; Ng et al, 2007; Ng et al, 2009; Rodriguez-Gabin et al, 2009). RAB31 overexpression is also implicated in cellular proliferation and apoptosis during cancer progression (Pan et al, 2015; reviewed in Chua and Tang, 2015). RAB31 is activated by VPS domain-containing GEFs GAPVD1 (also known as GAPEX5 or RAP6) and RIN3 (Hunker et al, 2006; Lodhi et al, 2007; Kajiho et al, 2003; Kajiho et al, 2011). Interaction of RAB31:GDP with its GEFs promotes release of GDP, allowing GTP to bind, and precludes the interaction of RAB31 with GDI and CHM proteins.