HPS4 is a component of the BLOC-3 (biogenesis of lysosomal organelles complex 3) protein complex, which functions as a guanine exchange factor for the small GTPases RAB32 and RAB38 1. The BLOC-3 complex, composed of one copy each of HPS1 and HPS4 that tightly associate via their longin domains, facilitates the conversion of RAB32/38 from inactive GDP-bound to active GTP-bound states, promoting their membrane localization 1. This complex is essential for biogenesis of lysosome-related organelles, particularly melanosomes, and controls melanin production 2. HPS4 mutations cause Hermansky-Pudlak syndrome type 4 (HPS-4), an autosomal recessive disorder characterized by oculocutaneous albinism, platelet storage pool defects with bleeding diathesis, and notably, progressive fibrosing interstitial lung disease (ILD) 34. HPS-4-related pulmonary fibrosis is a leading cause of mortality in affected patients, developing at younger ages than idiopathic pulmonary fibrosis and currently manageable only by lung transplantation, though nintedanib may provide temporary stabilization 35. Additionally, HPS4 variants show associations with cognitive function, particularly working memory in healthy individuals and executive function in schizophrenia 6.