STAT proteins are activated by diverse cellular stimuli including cytokine and growth factor receptor signaling, proto-oncogene and oncogene expression, and cellular stress mediators. In most cases, canonical STAT activation by a particular treatment or cellular condition results in STAT protein phosphorylation on an activating tyrosine residue near the C terminus. This phosphotyrosine is recognized by SH2 domains in partner STATs, resulting in homo- or hetero-dimerization. The STAT dimers attain the ability to bind specific DNA response element sequences present in the promoters of target genes. Two methods are described for the detection of activated STAT proteins based on (1) acquisition of tyrosine phosphorylation and (2) acquisition of DNA binding ability.
