SPP1 and Hermansky-Pudlak syndrome: In this study, we expanded our focus to include Osteopontin (OPN), Osteoprotegerin (OPG), soluble Receptor Activator of NF-κB Ligand (sRANKL), bone morphogenetic protein (BMP)-2, BMP-7, and Osteocalcin as additional quantifiable bone-related growth factors in HPS.