MAPK3 and angiosarcoma: Immunostaining of Erk1 and Erk2 proteins, which operate downstream from Ras, was barely detectable in each of three cell lines examined, and of the three canine hemangiosarcomas cell lines, only "Frog" showed evidence of constitutively active Erk1 and Erk2 based on positive staining by the presence anti-phospho-Erk antibody (Thr 202 and Tyr 204).