ABCB1 and acute lymphoblastic leukemia: While heightened levels of Pgp/ABCB1 have been associated with ALL relapse (Engle and Kumar, 2022), and overexpression of MRPs/ABCCs and BCRP/ABCG2 has been linked to resistance against various antifolates (Volk et al., 2002; Ansari et al., 2009; Liu et al., 2018; Jaramillo et al., 2019), our experiments found no significant differences in the expression of ABCB1, ABCCs, and ABCG2 between the Reh and Reh-MTXR cell lines.