Acquired third site mutations restore oncogenic activity to Kras^D12/G37 and Kras^D12/G64. (A) CFU-GM formation of fetal liver cells expressing WT Kras (●), Kras^D12 (▪), Kras^D12/G37/I50 (Δ), or Kras^{D12/G64/69RN70} (♢). Data show the mean of 3 independent experiments. (B) Representative CFU-GM morphology from fetal liver cells expressing Kras mutant alleles grown in 0.1 ng/mL GM-CSF. (C) Ras expression in GFP+, Mac1+ fetal liver cells infected with MSCV viruses encoding different Kras alleles. (D) Levels of pERK, pAkt, and pS6 in GFP+, Mac1+ fetal liver cells infected with MSCV viruses encoding different Kras alleles, as determined by flow cytometry using phospho-specific antibodies. Phospho-protein levels in cells expressing WT K-Ras were set at 1 in each experiment. Data shown are derived from 6 independent experiments.