RAS mutant alleles confer growth factor hypersensitivity to murine bone marrow cells. (A) Murine bone marrow cells were infected with retrovirus-expressing empty vector, wild-type KRAS, as well as each KRAS mutant allele. Cells were plated in triplicate in methylcellulose containing various doses of GM-CSF (0-10 ng/mL). After 7 days, colonies were quantified on a bright-field microscope. Values represent mean plus or minus SEM. (B) Murine bone marrow cells were infected with retrovirus-expressing empty vector, wild-type NRAS, and each NRAS mutant allele. Cells were plated in triplicate in methylcellulose containing various doses of GM-CSF (0-10 ng/mL). After 7 days, colonies were quantified on a bright-field microscope. Values represent mean plus or minus SEM.