Upon examining the effect of ABCB1 and ABCG2 on the toxicity and efficacy of citarinostat in human cancer cell lines, we found that the proliferation of cancer cell lines, regardless of the tissue of origin, was inhibited by citarinostat with the IC50 value ranging from 3 to 8 μM, which is similar to the value reported previously by Huang et al. [20] and North et al. [21]. This evidence concerns the gene ABCB1 and cancer.