English: Alternative splicing of HIV-1 tat exon 2. HIV-1 RNA is alternatively spliced in multiple ways. Equilibrium among differentially spliced transcripts, encoding different products, is required for viral multiplication. The inclusion of tat exon 2 in the RNA is regulated by competition between the splicing repressor hnRNP A1 and the SR protein SC35. Within exon 2 an ESS and ESE overlap. If A1 repressor binds, it initiates cooperative binding of multiple A1 molecules, extending into the 5’ donor site upstream of exon 2 and preventing the binding of U2AF35 to the polypyrimidine tract. If SC35 binds to the ESE, it prevents A1 binding and maintains the 5’ donor site in an accessible state for assembly of the spliceosome. Competition between the activator and repressor provides the required equilibrium among transcripts including and excluding exon 2.