Knockout of Rab27a/b in mice and Griscelli syndrome type 2 in humans, caused by mutation of the Rab27a gene, both show platelet defects resulting in increased bleeding times and a reduction in the number of dense granules, indicating that Rab27 is a key player in platelet dense granule biogenesis and secretion [9]–[12]. The gene discussed is RAB27A; the disease is Griscelli disease type 2.