Phospho-Stat3 (Tyr705) Sandwich ELISA Kit is a solid phase sandwich enzymelinked immunosorbent assay (ELISA) that detects endogenous levels of Phospho-Stat3 (Tyr705) protein. A Stat3 rabbit monoclonal antibody has been coated onto the microwells. After incubation with cell lysates, both nonphospho-and phospho-Stat3 proteins are captured by the coated antibody. Following extensive washing, a phospho-Stat3 mouse monoclonal antibody is added to detect the captured phospho-Stat3 protein. HRP-linked anti-mouse antibody is then used to recognize the bound detection antibody. HRP substrate, TMB, is added to develop color. The magnitude of optical density for this developed color is proportional to the quantity of phospho-Stat3 protein. Stat3 is a key signaling molecule for many cytokines and growth-factor receptors (1), and is required for murine fetal development (2). Additionally, Stat3 is constitutively activated in a number of human tumors (3,4) and possesses oncogenic potential (5) and anti-apoptotic activities (3). Stat3 is activated by phosphorylation at Tyr705, which induces dimerization, nuclear translocation and DNA binding (6,7). Transcriptional activation seems to be regulated by phosphorylation at Ser727, apparently via the MAPK or mTOR pathways (8,9). Stat3 isoform expression appears to reflect biological function. The relative expression levels of Stat3alpha (86kD) and Stat3b (79kD) depend on cell type, ligand exposure or maturation stage of the cells (10). It is notable that Stat3b lacks the serine phosphorylation site within the carboxy-terminal transcriptional activation domain (8).