Abstract： Objective To detect the expression of phosphorylated-signal transducer and activator of transcription 3 (p-Stat3) and myeloid leukemia-1 (Mcl-1) as well as their correlation,and to investigate the functional role of Stat3 and Mcl-1 in the pathogenesis of esophageal squamous cell carcinoma (ESCC).Methods Stat3 activity in ESCC cells was inhibited with JAK/Stat3 inhibitors (AG490 or JSI-124).Specific siRNA was used to inhibit the Stat3 expression.Cell apoptosis was detected by flow cytometry.Expression of Mcl-1 protein was determined by Western blotting.Expression of phospho-Stat3 (Tyr705) and myeloid leukemia-1 (Mcl-1) proteins in ESCC tissues was detected by tissue microarray and immunohistochemistry.The relationship between p-Stat3 or Mcl-1 aberrant expression and clinicopatholohical features of ESCC was analyzed.The correlation of their expression was also analyzed.Results Suppression of the Stat3 signaling activation in ESCC cells led to marked apoptosis,and dramatic reduction of Mcl-1 protein.The positive rate of phospho-Stat3 (Tyr705) expression was 45.0％ in 50/111 of the ESCC tissue samples.The lower the degree of tumor differentiation,the higher the positive rate of phospho-Stat3 (Tyr705),showing a significant difference (P =0.018).The positive rate of Mcl-1 protein expression was 72.1％ (80/111),and the lower the degree of tumor differentiation was,the higher there was the positive rate of Mcl-1,with a significant difference (P =0.026).There was a positive correlation between the expressions of p-Stat3 and Mcl-1 proteins (P =0.012).Conclusions In a subset of ESCC tissues,p-Stat3 (Tyr705) and Mcl-1 are overexpressed and positively correlated with each other,and both are correlated with tumor differentiation.Persistent activation of Stag3 contributes to apoptotic resistance in ESCC cells,and may be at least partly mediated through upregulation of Mcl-1.