Dose-dependent activation of ERK in ▵Raf-1:ER cells after stimulation with 4HT for 16 hours. ▵Raf-1:ER cells were stimulated at 37°C for 16 hours at 2 × 10⁵ cells/mL in RPMI, 10% FCS plus IL-3, IL-4, or IL-4 plus increasing concentrations of 4HT. ERK was IP from the cell lysate and kinase activity was determined in vitro by using MBP as a substrate. Phosphorylated proteins were visualized after SDS-PAGE and autoradiography. The quantity of IP protein in each lane was assessed by IB with antibodies against ERK (ERK).