SIGNAL transducers and activators of transcription (STATs) are activat
ed by tyrosine phosphorylation in response to cytokines and mediate ma
ny of their functional responses(1-3). Statil was initially cloned as
a result of its homology with Stat1 (refs 4, 5) and is widely expresse
d, although it is only tyrosine-phosphorylated after stimulation of T
cells with interleukin (IL)-12 (refs 6, 7). IL-12 is required for the
T-cell-independent induction of the cytokine interferon (IFN)-gamma, a
keg step in the initial suppression of bacterial and parasitic infect
ions. IL-12 is also important for the development of a Th1 response, w
hich is critical for effective host defence against intracellular path
ogens(8,9). To determine the function of Stat4 and its role in IL-12 s
ignalling, we have produced mice that lack Stat4 by gene targeting. Th
e mice were viable and fertile, with no detectable defects in haematop
oiesis, However, all IL-12 functions tested were disrupted, including
the induction of IFN-gamma, mitogenesis, enhancement of natural killer
cytolytic function and Th1 differentiation.