Transformation of NIH 3T3 fibroblasts by constitutively active M-Ras G22V. (A) NIH 3T3 fibroblasts were retrovirally transduced with (a) empty vector, (b) N-Ras Q61K, (c) M-Ras wt, or (d) M-Ras G22V. Shown are phase-contrast photomicrographs of cells grown in medium containing 10% calf serum. (B) Cell densities reached by polyclonal populations of NIH3T3 cells expressing vector alone (◊), M-Ras wt (□), M-Ras G22V (•), and N-Ras Q61K (▴). Cells were grown in 10% calf serum and figures represent data from 2 independent experiments, each performed in triplicate, and are shown as means ± SD.