Interleukin-6 (IL6) activates the tyrosine phosphorylation of STATs (Akira et al. 1994, Zhong et al. 1994) by receptor-associated JAKs (Hemmann et al. 1996) at a site that is essential for dimerization. For STAT1 this is tyrosine-701, for STAT3 tyrosine-705 (Kaptein et al. 1996, Shuai et al. 1994). Tyrosine phosphorylation leads to homo- or heterodimerization and translocation to the nucleus (Zhong et al. 1994), where the dimers bind to enhancers of IL6- inducible genes e.g. acute phase protein genes, resulting in transcriptional activation.