ABCB1 and cancer: It has been reported that other critical factors, such as 1) a decrease in the expression of topoisomerase (the cellular target of mitoxantrone (Harker et al., 1991); 2) mutations in the gene, ipt1, which encodes the expression of inositol phosphotransferase that is involved in sphingolipid biosynthesis in cancer cells (Ojini and Gammie, 2015); however, the role of ipt1 in mediating resistance to mitoxantrone resistance remains to be elucidated; and 3) the overexpression of other ABC transporters, e.g., ABCA2, ABCB1 or ABCC1 (Boonstra et al., 2004; Nieth and Lage, 2005).