Kikuchi et al. demonstrated that the knockdown of class IIa HDAC4 enhances cytotoxicity induced by the proteasome inhibitor carfilzomib in multiple myeloma via the upregulation of activating transcription factor 4 and the activation of endoplasmic reticulum (ER) stress-induced pro-apoptotic transcription factor C/EBP homologous protein [20]. Here, ATF4 is linked to plasma cell myeloma.