Upon an increase in P-gp activity, antitumor drugs that enter the cell can be actively pumped out of the cell (Dean et al., 2001; Kelland, 2007; Auner et al., 2010), leading to increased outflow and/or decreased inflow, so that the antitumor drugs do not accumulate at sufficiently high levels in tumor cells, thereby reducing their efficacy; this is the key mechanism by which P-gp exerts chemotherapy resistance (Kim et al., 2001; Genovese et al., 2017). The gene discussed is PGP; the disease is neoplasm.