Mutations in the U3 are frequent and unique to RNA interference (RNAi)-exposed virus. The frequency of mutation at each nucleotide (nt) position was calculated for the (a) long-terminal repeat (LTR) (nts −240 to +182) and psi (nts +243 to +353) and (b) tat (nts +5,376 to +5,591), rev (nts +5,516 to + 5,591), and vpu (nts +5,608 to +5,768) regions for each selective pressure (UP, unprotected; Comp., Compartmentalized; Equally, equally distributed). Several mutations in the U3 (a, nts −240 to 0) are present at high frequencies in virus exposed to RNAi. Mutations in tat exon 1 (b) are present at high frequencies, but not unique to RNAi-exposed virus. Nucleotides are numbered based on NL4-3 RNA. Ldr3 and TatB2 targets are located at nts +329 to +349 and +5,516 to +5,536, respectively.