Cysteine metabolism to its sulfoxidation end-products is dependent upon two iron-dependent enzymes that are the only known mammalian thiol dioxygenases. These two thiol dioxygenases are cysteine dioxygenase (CDO) and 2-aminoethanethiol dioxygenase (ADO, cysteamine dioxygenase ). Both of these thiol dioxygenases are essential for hypotaurine and taurine biosynthesis. ADO adds molecular oxygen to the sulfhydryl group of 2-aminoethanethiol (2AET, cysteamine) to form the sulfinic acid hypotaurine (HTAU) (Dominy et al. 2007).