EPO and acquired polycythemia vera: They concluded that, in PV, other factors besides erythropoietin (Epo) would play an important role in the production of erythroid cells, among them, IGF-1, which besides other effects, could interfere in apoptosis by increasing the expression of BCL-XL, elevated in PV, causing the accumulation of cells in the absence of erythropoietin(20).