In melanocytes Rab27a interacts with melanophilin to mediate the melanosome transport required for pigmentation.1 In natural killer (NK) cells and cytotoxic T lymphocytes, degranulation and thereby cell-mediated cytotoxicity require interaction of the active form of Rab27a with Munc13-4.2,3 Mutations in the human RAB27A gene cause the autosomal recessive Griscelli syndrome type 2 (GS2) with diluted pigmentation and immunodeficiency, which can progress to hemophagocytic lymphohistiocytosis (HLH).4 Recently, 6 patients with GS2 and normal pigmentation have been reported, identifying Rab27a mutations that selectively disrupt Munc13-4 binding when re-expressed in HEK293 cells.5 Here we present a new case of GS2 with no dilution of pigmentation and extend prior biochemical results through physiologic cell expression studies. Using NK and melanoma cell lines, we have investigated the binding activity of the mutations to the endogenously expressed interaction partners in comparison with wild-type Rab27a. ...we confirm the previously described biology in a truly physiologic context using cells that have full ability to mediate the functions of cytotoxicity or pigmentation.