CSTB and neurodegenerative disease: EPM1 is a neurodegenerative disease caused by bi-allelic partial loss-of-function mutations in the cystatin B (CSTB) gene (Pennacchio et al., 1996; Joensuu et al., 2008), a potent and reversible inhibitor of cysteine cathepsins (Turk and Bode, 1991).