Representative examples of suitable retroviral vectors and methods are described in more detail in the following U.S. patents and patent applications, all of which are incorporated by reference herein in their entirety: DNA constructs for retrovirus packaging cell lines, U.S. Pat. No. 4,871,719; Recombinant Retroviruses with Amphotropic and Ecoptropic Host Ranges, PCT Publication No. WO 90/028