AZD1480 treatment resulted in a reduction in both SP and PB MF HPCs. The inhibitory effect of AZD1480 on both SP and PB MF HPCs was assessed by determining a proportion of colonies generated in cultures of SP or PB MF CD34⁺ cells treated with cytokines or cytokines plus AZD 1480 in semisolid media. (A-B) The percentage of CFU-GM and BFU-E generated in the cultures of SP (A) or paired PB (B) MF CD34⁺ cells treated with cytokines plus AZD1480 relative to that generated in the cultures with cytokines alone. **P < .01; ***P < .001 (n = 10). (C-D) The percentage of JAK2V617F-positive (C) and JAK2V617F homozygous (D) colonies generated in cultures of SP or PB MF CD34⁺ cells containing cytokines plus AZD1480 or cytokines alone. Twenty-one to 35 individual colonies were randomly plucked and genotyped for the presence of JAK2V617F in each paired SP or PB MF CD34⁺ cell specimen cultured under each experimental condition from each of 4 patients. (E-F) The percentage of the absolute number of JAK2V617F-positive (E) and JAK2V617F homozygous colonies (F) generated in cultures of SP or PB MF CD34⁺ cells treated with cytokines plus AZD1480 relative to that generated in cultures treated with cytokines alone. **P < .01; ***P < .001 (n = 4). (G) The percentage of colonies having a chromosomal abnormality generated in cultures of SP or PB MF CD34⁺ cells treated with cytokines plus AZD1480 or cytokines alone. Twenty-one to 35 individual colonies were randomly plucked and genotyped for the presence of a chromosomal abnormality in each paired SP or PB MF CD34⁺ cell specimen cultured under each experimental condition from each of 4 patients. (H) The percentage of colonies having a chromosomal abnormality generated in cultures of SP or PB MF CD34⁺ cells treated with cytokines plus AZD1480 relative to that generated in cultures treated with cytokines alone. **P < .01; ***P < .001 (n = 4).