Anodonta woodiana belongs to the Unionidae that consists of freshwater bivalve mollusks and it is the main freshwater mussel in China.Theromaicn is an antimicrobial peptide that is an important component of the natural defenses of freshwater mussel.In this study,a theromacin gene from A.woodiana was identified and characterized.The full-length cDNA sequence of theromacin gene of A.woodiana was cloned by rapid amplification of cDNA ends(RACE).A 870 bp cDNA sequence contained a 306 bp open reading frame(ORF)encoding 101 amino acids,a 104 bp 5'untranslated region(5' UTR),and a 460 bp 3' untranslated region(3' UTR).The molecular weight of this mature peptide was estimated to be 11 1 66.9 u.Homology analysis indicated that amino acid sequence showed the highest similarity with theromacin in Hyriopsis cumingii(73%),and lower similarity with defensins,mytilins,myticins and mytimycins in mussels.It belongs to the macin family.Real-time quantitative PCR showed that theromacin gene expressed in a wide range of tissues including the mantle,blood,gill,foot,liver and intestine,with the highest level of transcripts in mantle,followed by foot,while in other tissues it was lowly expressed.After being challenged with Aeromonas hydrophila,the expression of theromacin gene was increased significantly in 6 tissues compared with the control mussel.The results suggested that theromacin gene might play an important role in the immune reaction of A.woodiana.