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Signal transducer and activator of transcription 3 (STAT₃) binds to Janus kinase 2 (JAK₂) to initiate the JAK₂/STAT₃ signal transduction pathway, which plays an important role in cancer cell proliferation, immune regulation, reproduction, lipid metabolism, and other physiological processes of the organism. In this study, the cDNA sequence of the STAT₃ gene from grass carp was cloned using RACE (rapid-amplification of cDNA ends). Twelve characteristics of the STAT₃ gene and its encoded protein sequence were predicted and analyzed using bioinformatics methods; these features included the general physical and chemical properties, the hydrophobicity, the secondary structure and the three-dimensional structure of the protein. Quantitative real-time PCR was employed to detect grass carp STAT₃ expression pattern in different tissues. The results showed that the full-length STAT₃ gene from grass carp is 2739-bp long and contains a 216-bp 5'UTR, a 300-bp 3'UTR, and a 2223-bp open reading frame (ORF) that encodes a 740-amino acid peptide. The deduced protein exhibited 99%∼94% homology to the STAT₃ protein of zebrafish (Danio rerio), medaka (Oryzias latipes), turbot (Scophthalmus maximus), white-spotted char (Salvelinus leucomaenis), mandarin fish (Siniperca chuatsi), rainbow trout (Oncorhynchus mykiss), and green pufferfish (Tetraodon fluviatilis). The deduced grass carp STAT₃ protein contains a protein interaction domain, an alpha domain, a DNA binding domain, and an SH2 domain. The STAT₃ protein of grass carp is a hydrophilic and non-secretory protein, and its molecular mass and isoeletronic point were found to be 98,5412.1 Da and 6.39, respectively. The structural elements of STAT₃ included α-helixes, β-sheets, and loops. The grass carp STAT₃ is expressed in all of the six tissues tested, which were the liver, spleen, gill, muscle, heart, and brain. The highest expression level was found in the liver (P < 0.05), whereas a significantly lower expression level was found in the spleen, gills, brain, and muscle (P < 0.05), and the lowest expression level was found in the heart (P < 0.05). This study provides a basis for further structural and functional exploration of the STAT₃ from grass carp, including its deduced protein and its signal transduction function., (Copyright © 2013 Elsevier Ltd. All rights reserved.)