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.