CTSB and cancer: CTSB is not only produced in response to muscle activity, but for instance also by macrophages during perineural invasion (Bakst et al., 2017), cartilage cells (Zwicky et al., 2002), by various types of cancer, where high CTSB expression often goes along with a less favorable prognosis (Chan et al., 2010; Ozeki et al., 1993; Ruan et al., 2016), and by microglia in the central nervous system (Ni et al., 2019).