NPTN and infection: Of note, infection with SFFV alone, which was observed far more frequently than double infection in the early phase of FV infection, is sufficient for the induction of erythroblast proliferation, since SFFV gp55 is also expressed in the absence of F-MuLV and will by itself interact with and activate the erythropoietin receptor and sf-Stk to induce erythroleukemia [29].