Phospho-Stat3 (Tyr705) refers to the phosphorylated form of Signal Transducer and Activator of Transcription 3 (STAT3) at the tyrosine 705 residue. Phosphorylation of STAT3 at Tyr705 is a critical step in the activation of the STAT3 signaling pathway. This phosphorylation event, often induced by cytokines, growth factors, and oncogenes, leads to the formation of STAT3 dimers, which then translocate to the nucleus to initiate gene transcription. STAT3 is involved in various cellular processes, including cell growth, survival, and immune responses. Phospho-Stat3 (Tyr705) is essential for mediating cellular responses to interleukins, KITLG/SCF, LEP, and other growth factors. In the context of immunity, STAT3 plays a crucial role in regulating the differentiation of naive CD4(+) T-cells into T-helper Th17 or regulatory T-cells (Treg), which is controlled by the phosphorylation at Tyr705. Abnormal activation of STAT3, including constitutive phosphorylation at Tyr705, has been observed in various cancers and is associated with tumor growth, survival, and angiogenesis. In some cases, STAT3 activation is driven by autocrine secretion of interleukin-6 (IL-6), leading to continuous activation of the Jak/Stat pathway.