OTC and neoplasm: Distinct cell types in the tumor microenvironment (TME) and T follicular helper (Tfh) tumor cells, the origin of AITL, were identified according to our previous study (Figure S3A, Supporting Information).[45] We first analyzed the single‐cell expression of ASS1, ornithine transcarbamoylase (OTC), arginase2 (ARG2), and argininosuccinate lyase (ASL), four key enzymes in the urea cycle that produce arginine during the detoxification of excess ammonium,[46] as shown in Figure S3B (Supporting Information).