CML CFC (A) and LTC-IC (B) are at least as sensitive to MTX as their normal counterparts. Five thousand normal and CML CD34⁺HLA-DR⁺ cells were plated in serum-free methylcellulose assay with increasing MTX concentrations to determine the MTX sensitivity of CFC (left panel). Alternatively, 10,000 normal CD34⁺HLA-DR⁻ cells (LTC-IC) and CML CD34⁺HLA-DR⁺ (Ph⁺ LTC-IC) cells were incubated for 1 week in serum-free medium, IL-3, IL-6, and SCF with increasing MTX concentrations. Cells were then plated in contact with M2-10B4 stromal feeders for 5 weeks. The number of MTX resistant LTC-IC was determined by replating LTC derived progeny in methylcellulose assay without MTX.