XBP1 and inclusion body myositis: ER-stress and the activation of the unfolded proteins response (UPR), as the respective cellular defence mechanism, have been described in sporadic inclusion-body myositis (sIBM), In contrast, UPR key players (ATF4, ATF6, BiP and XBP1) in muscle of GNE patients lacked any evidence of UPR induction.