In addition, studies have shown that some natural compounds, such as menthol, combine with non-toxic digitalis glycosides to enhance cytotoxicity and inhibit cancer cells proliferation by downregulating the expression of ATP binding cassette subfamily B member 1 (ABCB1) (Eid et al., 2012; Eid et al., 2013). This evidence concerns the gene ABCB1 and cancer.