Efficiency and specificity of siRNA-mediated inhibition of v120-A and v126-D replication. Panels A and B illustrate the specificity of siRNA120a for the 5' end of v120-A and of siRNA126a for the 3' end of v126-D. As described in Gao et al. 2008 , these siRNAs show specificity based on complete complementarity with the HIV-1 target sequence (nt 6415-6435 for siRNA120a and nt 8120-8140 for siRNA126a) of the specific HIV-1 isolate. The ability of the siRNAs to inhibit v120-A and v126-D is shown in panels C and D. Panel C is a reproduction of a previous experiment presented in Gao et al.  and shows the inhibition of HIV-1 v120-A and v126-D by siRNA120a. Panel D is showing the specificity of siRNA126a for inhibition of HIV-1 v126-D as opposed to v120-A. Virus production was monitored by RT activity in supernatant at day 5 post-infection and presented relative to the no drug control (NC) (RT values are 1937 and 1852 cpm/ml for v120-A and v126-D, respectively). The IC50 value of siRNA120a for inhibition of v120-A was approximately 0.16 nM and 0.021 nM for the IC50 of siRNA126a for inhibition of v126-D.