Impaired proliferation and survival of Stat5a/b-null GMPs cultured with GM-CSF. Time-lapse microscopy and single-cell tracking were used to determine generation times and survival rates of GMPs and their progeny cultured in the presence of GM-CSF through 5 generations. (A) Typical GMP pedigrees generated by single-cell tracking. X represents cell death. (B) STAT5A/B-dependent survival. Initiating GMPs are referred to as parental cells (P). (C) Average generation time of all cells within 1 generation. For each experiment, more than 30 initiating GMPs and their progeny were tracked up to the fifth generation. Data are mean ± SD of 3 independent experiments (n > 30 pedigrees per experiment). **P < .01 in panels B and C.