Comparative activity of imatinib and sorafenib for inhibition of KIT kinase and activation of downstream signaling pathways in GIST cell lines. GIST cell lines were treated with imatinib or sorafenib for 6 hours and then harvested for protein lysates. Whole-cell lysates were immunoblotted and the membrane was probed with antibodies to activated (p-KIT, p-AKT, and p-S6) and total forms of KIT, AKT, and S6. The bottom frame contains the results for β-actin, which was used as a loading control.