Mechanism of Friend virus-induced erythroblastosis and polycythemia. gp55 of the anemia-(gp55_A) or polycythemia-inducing (gp55_P) strains of Friend virus activates sf-STK, causing uncontrolled erythroid proliferation and erythroblastosis. gp55_P activates the EPOR, JAK2, and STAT5, causing deregulated erythroid differentiation and polycythemia.