Mesallamy et al. examined the effect of the ABCG2 421C>A (rs2231142) mutation on plasma levels of MTX in Egyptian pediatric ALL patients, and they showed that there was no correlation between the ABCG2 421C>A and MTX plasma levels (El Mesallamy et al., 2014). Here, ABCG2 is linked to acute lymphoblastic leukemia.