Cysteine proteases, also known as thiol proteases, are enzymes that degrade proteins. The disulfide link is readily reduced to give the corresponding thiol cysteine. They are the product of esterification between a carboxylic acid and a thiol. The reaction results in an anti-Markovnikov addition of a thiol compound to an alkene. Organo-triethoxysilane molecules, either thiol or vinyl tailed, have been introduced in surface functionalization. This reagent reacts with electrophiles to give thienyl derivatives, such as the thiol. NCL reaction is catalyzed by in situ transthioesterification with thiol additives. Thioglycoligases catalyze the condensation of activated glycosides and various thiol containing acceptors. The albumin Cys34 thiol exists in both reduced and oxidized forms. The carbon-centered radical can undergo chain-growth polymerization depending on the thiol and ene functional groups. Generally, DTT is used as a protecting agent that prevents oxidation of thiol groups. The alkylthiosulfates are susceptible to hydrolysis, affording the thiol. Glutathione is the most abundant thiol in animal cells, ranging from 0.5 to 10 mM. Some proteins test positive when denatured, indicating that thiol groups are liberated. This foul-smelling colorless liquid is the simplest aromatic thiol.