In mice there are at least 16 naturally occurring hypopigmentation models of Hermansky-Pudlak syndrome (HPS), and 9 of these have been characterized at the molecular level. Zhang et al. (2003) used positional candidate and transgenic rescue approaches to identify the genes mutated in ruby-eye-2 (ru2) and ruby-eye (ru), 2 'mimic' mouse models of HPS. They determined that these genes are orthologs of the human genes mutated in individuals with HPS5 and HPS6, respectively. Both genes are found only in higher eukaryotes. The human ru ortholog contains 775 amino acids and is 80% identical to the mouse protein. Northern blot analysis of mouse tissues detected a 2.6-kb ru transcript in all tissues tested, with lowest expression in skeletal muscle.