Quantitation of LTC-IC by limiting dilution analysis. Four dilutions of CD34⁺ EGFR⁺ cells (•) and CD34⁺ EGFR⁻ cells (□) were cultured over irradiated stromal layers, and the number of clonogenic cells detectable after 7 weeks was determined. In this experiment, the frequency of LTC-IC was 1:768 cells in CD34⁺EGFR⁺ cells (—) and 1:1,206 in CD34⁺EGFR⁻ cells (---).