Type I interferons (IFN-ls) activate Stat4 and promote Th1 development in h
uman, but not mouse, CD4(+) T cells. Recently, IFN-1-induced Stat4 activati
on in human cells was found to require interactions of Stat4 with Stat2. A
remarkable genetic dissimilarity between murine ann human Stat2 explains th
e ability of human, but not mouse, CD4(+) T cells to undergo IFN-1-induced
Stat4 activation and Th1 development.