Activities of different shRNAs from different vectors. (a) The first two test hairpins, Tat 56-29 (T) and Vif 88-29 (V), were expressed from separate plasmid vectors (pS) via the H1 polymerase III promoter. HEK293a cells were transfected with 800 ng of the pSilencer-based (pS) T vector (+ empty) (b), the V vector (+ empty) (c), or the T and V vectors together (d) at mass ratios of 1:7 to 7:1, plus 100 ng each of both target-specific reporter vectors GFPsTat (T.r) and AsRed1sVif (V.r-red). There were 9 data points plotted for each curve, though for clarity open circles are only shown on every 2nd one. The data shown is representative of several replicated experiments (repeated twice, i.e. n = 2).