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A novel sialic acid-specific lectin (MCsialec) was detected from an expressed sequenced tag (EST) sequence from manila clam hemocytes infected with Perkinsus olseni (Kang et al. 2006, Dev. Comp. Immunol. 30, 1119–1131). The cDNA of the lectin was cloned using gene-specific primers based on a previously determined EST and characterized. The full-length cDNA of MCsialec is 603bp in length and encodes a polypeptide of 200 amino acids with a calculated molecular mass of 21.928 kDa. Sequence alignment and protein motif analyses showed that MCsialec shares identity with sialic acid-specific invertebrate lectins from Cepaea hortensis, Helix pomatia, and Haliotis discus discus. The lectin was expressed in E.coli M15cells and purified using a Ni-NTA His-binding resin matrix for antibody production. The presence of the lectin in various tissues of Perkinsus-infected and -uninfected manila clams was analyzed by both PCR and immunohistochemical localization assays. MCsialec was detected in each tissue of the clams; however, upon infection, the level of expression of the lectin increased in each tissue. Vibrio tapetis infection also induced high-level expression of MCsialec in the hemocytes. These data suggest that MCsialec plays a crucial role in the immune system of the manila clam during pathogenic infection.