Raf-1 and Fas have opposite effects on MAPK signaling during the differentiation of primary fetal liver-derived erythroblasts. (A) Phosphorylation of ERK, JNK, and p38 as well as the amount of ERK, JNK, p38, and ASK1 expressed by differentiating erythroblasts were determined by immunoblotting. The experiments were reproduced 4 times with comparable results. (B) ASK1 mRNA increases during erythroblast differentiation in a Fas-dependent manner. RT-PCR was repeated twice.