ASS1 and neoplasm: Human T cells do not express ASS1 (Ohno et al., 1992; Sugimura et al., 1990), and chimeric antigen receptor (CAR)-T cells engineered to express ASS1 show enhanced proliferation and in vivo anti-tumor activity (Fultang et al., 2020), indicating that inducing expression is a mechanism to tolerate arginine depletion.