Rapid, stable suppresion of NDR by retrovirus-delivered siRNA HeLa cells were infected with the viruses illustrated in Fig. 1. Proteins were extracted from cells infected with each virus and NDR levels were examined by western blot analysis. Arrow indicates dramatic downregulation of NDR only in cells infected with MSCV/U6-NDR. The upper band was detected by the anti-p75 antibody, which serves as an internal loading and specificity control. Identical results were obtained with cells examined 24-hours post infection or after several passages (data not shown).