MBTPS1 and osteogenesis imperfecta: Even though S1P mutations in humans are yet to be reported, mutations in SREBP-2 (a major S1P/S2P substrate) are linked to osteoarthritis pathogenesis in humans (Kostopoulou et al., 2012), and mutations in S2P are linked to osteogenesis imperfecta (Lindert et al., 2016).