IL-12 signaling through STAT4 is essential for induction of optimal levels of IFN-γ production and commitment of Th1 cells. The molecular mechanism that controls how IL-12 and STAT4 signaling induces Th1 differentiation is poorly described. To identify the early target genes of IL-12 and STAT4 signaling, oligonucleotide arrays were used to compare the gene expression profiles of wild-type and STAT4-knockout murine Th cells during the early Th1 differentiation. According to the results, 20 genes were regulated in an IL-12-and STAT4-dependent manner. Importantly, Ifnγ was clearly the first gene induced by IL-12 in a STAT4-dependent manner. Most of the other defects in gene expression in STAT4-knockout cells were seen after 48 h of Th1 polarization. In addition to IL-12 signaling mediated by STAT4, STAT4-independent induction of a number of genes was observed immediately in response to Th1 induction. This induction was at least in part driven by IFN-γ independently of STAT4. Importantly, addition of exogenous IFN-γ into Th1 cell cultures of STAT4-knockout cells restored the defect in IFN-γ production further demonstrating the critical role of IFN-γ in early Th1 differentiation.