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Using a cDNA library prepared from venomous glands of the Mexican scorpion Centruroides noxius Hoffmann the genes that encode toxins 1 and 2 were identified, cloned and sequenced. In view of the proposed mechanism for processing the mature peptides coded by these two genes, the corresponding peptide-toxins were sequenced de novo. Mass spectrometric and 1H-NMR analyses of the C-terminal peptide produced by enzymatic digestion of both toxins indicated that the last residue is serine-amide. Sequence comparison revealed that these two genes have a similarity of 56% and 80% at the amino acid and nucleotide levels, respectively. Small corrections to the published primary structures were introduced: Cn toxin 1 has an extra serine residue at position 65 and the residue in position 60 is a proline, while the amino acids at positions 34 and 35 of Cn 2 are, respectively, tyrosine and glycine. Sequence comparison of toxins from the genus Centruroides suggests the presence of at least three classes of distinct peptides in these venoms.