HIV-1 Tat induces the expression of interferon (IFN)-inducible genes in immature dendritic cells (iDC) in the absence of IFN production. We evaluated how three alleles of Tat and some Tat mutants differ in cellular gene modulation and whether a similar gene induction pattern could be detected by treating cells with IFNs. The three alleles and mutants, with the exception of mutants TatSF21-47 and TatSF2G48-R57A that do not localize in the nucleous, modulated to different degrees IFN-inducible genes without concomitant induction of IFNs. The first exon TatSF21-72 and the minimal transactivator TatSF21-58, all induced genes to a significantly greater extent than full-length Tat. The 2nd exon appears to diminish the gene modulation that can be observed when the first exon alone is expressed.