18 illustrated by the fact that mutations of IL-12 or its receptor interfere with cell-mediated immunity in mice and humans. Type 1 IFNs can also promote TH1 differentiation. However, despite the fact that many other aspects of the control of TH development are well conserved between mouse and man, this aspect of regulation evidently is not: type 1 IFNs were found to induce IFN-γ production in human but not mouse cells5, 6. The transcription factor STAT4 is pivotally involved in lymphocyte responsiveness to IL-12 and TH1 differentiation7. A clue to the mechanism for the species-specific difference in the immunoregulatory effects of type 1 IFNs emerged when it was found that in addition to IL-12, type 1 IFNs also induced phosphorylation of STAT4 (ref. 8). Although this occurred in human lymphocytes, it did not occur in mouse cells9. The question was, why did this response not occur in mouse lymphocytes? Murphy and colleagues investigated this problem and showed that type 1 IFN-induced STAT4 phosphorylation is dependent upon STAT210. In fact, they found that murine STAT4 was phosphorylated in response to type 1 IFNs, when transfected into cells that expressed human STAT2. So it was not something intrinsic to murine STAT4 that accounted for its lack of phosphorylation in mouse cells. Rather, it was noted that there is relatively poor sequence identity between human and mouse STAT2. This led the authors to hypothesize that mouse STAT2 might be unable to recruit and activate mouse STAT4. On page 65 Farrar et al. 2 confirm this hypothesis and describe a molecular basis for the difference in type 1 IFNs pathways between humans and mice. Using the well characterized STAT2-deficient human fibrosarcoma cell line, U6A, the authors show that the COOH-terminus of human STAT2, which contains two key tyrosine residues (Y833 and Y841), is responsible for recruiting and activating STAT4. As might be expected, the activation of STAT4 is dependent upon its SH2 domain, presumably via binding to phosphorylated Y833 or Y841. This is in sharp contrast with the means by which STAT1 is recruited—phosphorylation of STAT2 on Y690 being essential for STAT1, but not STAT4, activation. In the mouse Stat2 gene, exon 23 has 12 copies of a 24-nucleotide minisatellite insertion. These insertions disrupt the ability of mouse STAT2 to recruit STAT4. But if this segment is replaced with the COOH-terminal portion of the human STAT2 gene, STAT4 activation can occur. These discoveries explain why type 1 IFNs fail to activate STAT4 in the mouse. Given the importance of STAT4 in TH1 development and IFN-γ production, the paper by Farrar et al. provides a clear mechanism for species-specific effects of type 1 IFNs. Additionally, these discoveries represent a novel means of STAT recruitment to the cytokine receptors, although it is not resolved whether this is directly or indirectly mediated. The result is also notable because, with the exception of STAT1, little has been done to map STAT phosphorylation sites. Certainly, it was not expected that the STAT2 COOH-terminal tyrosines (Y833 and Y841) would be functionally relevant sites of phosphorylation. This latter point has not been established formally, however, as the authors used various mutants but did not directly assess STAT2 phosphorylation. Even so, these results go a long way to explain the difference in type 1 IFNs action in human versus mouse cells. One obvious caveat is that the system used is artificial; STAT4 is not physiologically expressed in the U6A cell line, the authors needed to transfect it in. Presumably, the reconstitution experiments reflect the physiologic circumstance, but we cannot …