Inverse targeting by SCF and EGF displaying retroviruses. When a mixed population of A431 (Kit-negative, EGF-R–positive) and TF-1 cells (Kit-positive, EGF-R–negative) was infected by vectors incorporating either the SCFXA or the EXA chimeric envelopes, the SCFXA vectors preferentially infected the Kit-negative cells and the EXA vectors preferentially infected the EGF-R–negative cells. A photomicrograph of a representative experiment is shown here. Upon X-gal staining, infected cells turn blue.