In panel A, WT2 cells were incubated in the absence and presence of 10 μM U0126 for the indicated times, and then treated with or without PMA for 15 minutes. Whole-cell extracts, equalized for protein, were immunoblotted for phospho-Erk and total Erk. In panel B, WT2 cells were incubated in the absence and presence of 10 μM U0126 for 2 hours, and then treated with PMA for the indicated times. Whole-cell extracts, equalized for protein, were immunoblotted for phospho-Raf (Ser259, Ser289/296/301, Ser338), phospho-Erk and total Erk.