APEX1 and Miyoshi myopathy: MDR1 is constitutively expressed to a higher level in melphalan-resistant MM cells as shown in Figure 6A. Since a previous study[18] reported that APE1 plays a critical role in regulation of MDR1 expression through a novel acetylation modification, we then postulated that APE1 could be involved in melphalan resistance in MM cells by regulating MDR1 expression.