ATP2A1 and inclusion body myositis: Robust (~4-fold) elevations in autolytic activation of the Ca2+-activated protease calpain-1 in IBM provides strong evidence of abnormally high cytosolic Ca2+ concentration, and may explain specific protein expression decreases (e.g. SERCA1, SERCA2, and RyR1) that were out of proportion with their respective mRNA decreases.