NBMPR, also known as S-4-nitrophenylmethyl-6-thioinosine, is a nucleoside analog that competitively inhibits the equilibrative nucleoside transporter 1. NBMPR is routinely used at concentrations of 0.10 μM and 0.10 mM to specifically inhibit transport of nucleosides mediated by equilibrative nucleoside transporters 1 (ENT1) and 2 (ENT2), respectively. NBMPR at a concentration of 0.10 mM abolishes ABCG2 activity.