S-Aminoethyl-l-cysteine, also known as thialysine, is a toxic analog of the amino acid lysine in which the second carbon of the amino acid's R-group (side chain) has been replaced with a sulfur atom. S-Aminoethyl-l-cysteine, also known as thialysine, is a toxic analog of the amino acid lysine in which the second carbon of the amino acid's R-group (side chain) has been replaced with a sulfur atom.