Suppression of NDR and p75 expression by retrovirus-delivered siRNA. HeLa-CD4 cells were infected the viruses illustrated in Fig. 1 plus an additional virus, MSCV/U6-p75. Western blot analysis demonstrates a dramatic downregulation of p75 in cells infected with MSCV/U6-p75 and NDR in cells infected with MSCV/U6-NDR. Infection with the various control viruses (MSCV, MSCV/U6, or MSCV/U6-cont) had no effect on p75 or NDR levels. For the mock-infected sample, puromycin selection was omitted.