A. IC50 values for the selective MEK1/2 inhibitor PD0325901 for thirty-five colorectal, lung and breast cell lines including all seven models harboring exon 4 KRAS mutation (4 A146T, 2 A146V and 1 K117N). B. Representative IC50 curves for the LS1034 (A146T KRAS), C80 (A146V KRAS), CCCL-18 (A146T KRAS, PIK3CA E542K), HCT-15 (KRAS G13D, PIK3CA E545K) and COLO-205 (V600E BRAF) models. C. Treatment of A146T KRAS expressing cell lines with PD0325901 was associated with a decrease in pERK and cyclin D1 expression, induction of p27 and hypo-phosphorylation of RB. (D). Cell lines expressing exon 4 KRAS mutations were resistant to the EGFR inhibitor gefitinib. The gefitinib-sensitive H3255 (L858R EGFR) model is included for comparison.