The function of HPS4 (HPS4 biogenesis of lysosomal organelles complex 3 subunit 2, Ensembl gene identifier ENSG00000100099) is as follows. Component of the BLOC-3 complex, a complex that acts as a guanine exchange factor (GEF) for RAB32 and RAB38, promotes the exchange of GDP to GTP, converting them from an inactive GDP-bound form into an active GTP-bound form. The BLOC-3 complex plays an important role in the control of melanin production and melanosome biogenesis and promotes the membrane localization of RAB32 and RAB38 (PubMed:23084991). .