ASS1 and cancer: Because some cancer cells were demonstrated to lose enzymes to synthesize specific non-essential amino acids (such as arginine, by cells lacking argininosuccinate synthase (AS-Sinth), which is indispensable for recycling citrulline to arginine) [51] while implementing the synthesis some NEAA, even if abundant in food and in circulating fluids, such as serine/glycine, the hypothesis was that cancer spares NEAA synthesis because it develops adapted to NEAA concentrations most abundant than EAA, as normally found in biological fluids.