During the past two decades, many investigators have assumed that the major locus of regulation of cysteine catabolism is the partitioning of cysteinesulfinate between its decarboxylation and transamination pathways. Hepatic cysteinesulfinate decarboxylase activity correlates well with the capacity of animals to synthesize taurine1–4, and low cysteinesulfinate decarboxylase activity in the cat has been associated with its nutritional requirement for dietary taurine5. More recent studies in our laboratory have indicated that cysteinesulfinate-independent pathways also play a major role in cysteine metabolism6,7. In contrast to cysteinesulfinate-dependent metabolism of cysteine, which leads to both taurine and sulfate production, the cysteinesulfinate-independent pathways all result in release of reduced inorganic sulfur and its subsequent oxidation to sulfate. This evidence revealing a contribution of cysteinesulfinate-independent pathways to cysteine catabolism suggested that partitioning of cysteine between cysteinesulfinate formation and metabolism by cysteinesulfinate-independent pathways may also be important in the regulation of cysteine metabolism to taurine.