EPO and infection: Erythroid progenitors from Fv2 resistant (i.e. sf-Stk null) mice, which do not form erythroid colonies in vitro in the absence of Epo, are able to form Epo-independent colonies following SFFV infection if they are first engineered to express sf-Stk [49], or if they are infected with a bicistronic retroviral vector allowing coexpression of SFFV Env and sf-Stk [67].