OTC and cancer: Most cancer cells are arginine-auxotrophic, meaning they lack arginosuccinate synthetase (ASS) or ornithine transcarboxylase (OTC) expression in the urea cycle to recycle arginine; thus, arginine depletion therapy can be used to treat a broad spectrum of cancer cells, including melanoma, colorectal, lung, ovarian, and hepatocellular carcinoma [6–15].