These contradictory observations have been previously reported by other authors (Di Pietro et al., 2002; Deferme and Augustijns, 2003) and could be due to the different concentrations used in the experiments (Yuan et al., 2012), the type of cancer cells tested (Di Pietro et al., 2002) and differences in the interactions between the flavonoids and P-gp depending on the substrate used in the experiments (Di Pietro et al., 2002; Eid et al., 2015). This evidence concerns the gene PGP and cancer.