As shown in Figure 4C, rifabutin (2.5 μM) and verapamil (10 μM) produced similar sensitization effects in cells co-treated with VIC, suggesting that a low dose of rifabutin could increase the cytotoxicity in VIC-treated KBV20C cells to levels similar to that obtained with verapamil in sensitizing drug-resistant cancer cells overexpressing P-gp. The gene discussed is PGP; the disease is cancer.