Mutations within the genes of Rab27a, hMunc13-4, Syntaxin11, and Munc18-2 lead to defective CTL and NK cell function resulting in often lethal immune disorders: Griscelli syndrome (Rab27a) and FHL type-3, -4, and -5 (hMunc13-4, Syntaxin11, and Munc18-2, respectively). Here, RAB27A is linked to Griscelli syndrome.