Model of erythroblastosis and polycythemia induced by Friend virus. The envelope glycoprotein (gp55) of both the anemia (A)- and polycythemia (P)-inducing strains of SFFV induces erythroblastosis and splenomegaly that requires sf-STK. Only gp55_P induces Epo-independent terminal erythroid differentiation, through EpoR and Stat5. For details, see the complete figure in the article beginning on page 73.