OSTM1 and osteopetrosis: In spite of normal lysosomal pH, lysosomal degradation of endocytosed protein is impaired in ClC-7/Ostm1-deficient mice (Wartosch et al., 2009), which develop a neurodegenerative lysosomal storage disease in addition to osteopetrosis (Kasper et al., 2005; Lange et al., 2006; Pressey et al., 2010).