A cDNA library of Limax flavus was constructed and screened for sialic-acid-specific lectins. Complementary DNA clones were categorized into seven groups corresponding to closely related but different sequences. Group 1 clones contained an ORF encoding 199 amino acids including a sequence identical to the partial amino acid sequence obtained from the lectin protein. Within its 1074-bp 3' untranslated region, ten closely related 60-bp sequence repeats were found. Group 2 clones contained an ORF encoding a polypeptide chain of the same number of amino acid residues, with 89.1% overall identity to that of the group 1 and eight 60-bp repeat sequences in the 3' untranslated region. The remaining groups of clones contained ORF with highly similar full or partial sequences, with or without 60 bp repeats in the 3' untranslated region. The large number of closely related but different cDNA clones obtained indicated that the slug sialic-acid-specific lectin gene is a member of a multigene family. The lectin amino acid sequence showed significant similarity with the fibrinogen domain of human tenascin-C, with a human C-type serum lectin, and with pig ficolin. Immunostaining analysis of slug tissue for the lectin indicated that it is present primarily on the epidermal surface and in mucous glands. Recombinant slug lectin protein lacking the 20-amino-acid N-terminal signal sequence produced in a bacterial expression system from a group-1 clone accumulated as aggregates in inclusion bodies, suggesting that large-scale production of the active agglutinin may be possible.