SOS1 and phospho-VAV1 bound to GRB2 in a complex with the activated EPOR catalyze the exchange of GDP for GTP bound to RAS proteins, yielding RAS:GTP (Komatsu et al. 1992, Torti et al. 1992). RAS:GTP then activates RAF1 and ERK1 and ERK2. EPO is observed to activate HRAS (Komatsu et al. 1992, Torti et al. 1992), KRAS, and NRAS (inferred from mouse homologs). KRAS appears to be particularly important for erythropoiesis since deletion of KRAS causes anemia in mouse.