NPTN and polycythemia: SFFV-encoded gp55 glycoprotein binds and activates the EPOR bypassing the cellular requirement for EPO and supporting proliferation and survival of infected erythroid cells.61 To address the leukemogenic potential of a constitutively active EPOR, Longmore and Lodish48 generated an SFFV in which they replaced gp55 with a mutated EPOR. Injection of this modified SSFV-induced polycythemia and splenomegaly in mice.48 Transfer of growth-factor-independent erythroblasts isolated from the spleens of these mice rapidly induced an erythroleukemia-like disease in the recipients.