Escape mutations in the 19-nt HIV-1 integrase target region. The 19 nt target is shown. Mutations were scored in multiple evolution cultures. The frequency of each escape mutation is listed in the middle column (marked gray). Amino acid changes in the integrase enzyme are shown in the right column. The upper panel shows the escape profile on the target sequence induced by shRNA-wt (21 cultures from  and 2 from this study). The lower panel shows the more restricted escape profile for shRNA-combi (wt+G8A+G15A) observed in 21 cultures.