RAB39B and Parkinson disease: Rab39B, on the other hand, is a brain-enriched Rab whose mutations have been implicated in X-linked intellectual and developmental disability (Giannandrea et al., 2010; Andersen et al., 2014) and early-onset Parkinson’s disease (PD; Wilson et al., 2014; Lesage et al., 2015; Mata et al., 2015).