Relation between the ratio of intracellular MTX-PG to extracellular plasma MTX concentration (Cp_ss) versus chromosome 21 copy number in ALL blasts. (A) Data for B-lineage ALL blasts isolated from bone marrow at 44 hours after low-dose MTX treatment of 60 children (n = 43, 7, and 10 for 2, 3, and 4 copies of chromosome 21, respectively). (B) Data from B-lineage ALL blasts isolated from bone marrow at 44 hours after high-dose MTX treatment in 61 children (n = 42, 10, and 9 for 2, 3, and 4 copies of chromosome 21, respectively). (▪) Mean values; the boxes depict the standard errors of the mean; and bars depict the range of ±1 standard deviation in each group.