HSPC differentiation is blocked to a greater degree by Myc transduction than by Myc^V394Dtransduction. c-Kit⁺ HSPCs were transduced with MSCV-GFP, Myc-GFP, or Myc^V394D-GFP, respectively. Transduced Kit⁺ HSPCs were purified by FACS 2 days posttransduction and incubated in HSPC culture medium with medium change every other day (A-F) or seeded into methylcellulose for serial replating clone-forming assay (G). Proliferation was examined by BrdU pulse-labeling assay (A), Ki67 staining assay (B), and cell-cycle analysis (C) at the indicated number of days in culture. Cell differentiation was analyzed by flow cytometry to examine c-Kit and Gr1 expression (D-E) and by morphology to count the percentage of leukemic blasts on indicated days (F). The numbers of CFUs were counted 7 days after each plating (G). *P < .05; **P < .01. ns, not significant; P1/P2/P3/P4, plating 1/plating 2/plating 3/plating 4.