Mutation of M1 by a single, silent point mutation does not alter vif mRNA expression within an infectious molecular clone. HIV-1 splicing pattern of PM1 cells infected with NL4/3 or NL4/3 carrying a silent point mutation within M1 (ΔM1-43, cf. Fig. 8A). The upper part shows an RT-PCR amplification for the vif 1.2I mRNA (the letter I denotes incompletely spliced or intron-containing mRNA) using primer pair #1544/#2183. The middle part was obtained using primer pair #1544/#1542. The stars indicate that these bands were isolated from the gel and confirmed by sequencing. To compare the total amount of HIV-1 transcripts within both mRNA preparations a separate RT-PCR was performed using primer pair #2335/#480 amplifying exon 1 sequence. (B) RT-PCR analysis of RNA from HeLa-T4+ cells transfected with the minigene lacking D2 (cf. figure legend 8B).