HDAC9 and cancer: It is worth mentioning that several studies have reported that prolonged exposure of cancer cells to some HDAC inhibitors can lead to the induction of ABCB1 and/or ABCG2 and the MDR phenotype [64,65,66,67,68,69,70], and the overexpression of ABCB1 and/or ABCG2 has also been linked to reduced efficacy of numerous HDAC inhibitors [30,39,40,68,70,71,72].