(Written for the original Russian edition) Out of all the functional groups of protein molecules that be- long to the side chains of amino acid residues, the sulfhydryl or thiol (SH) groups of cysteine residues and the disulfide (S-S) groups of cystine have for long attracted particular attention from chemists and biochemists. This is due, on the one hand, to the high chemical reactivity of these groups, which enter into many, and often very characteristic and selective, reactions with several types of compound. On the other hand, this attention is connected with the great significance of SH...