Macrophages in tumours predominantly have an M2‐like phenotype and contribute to immune suppression and T cell dysfunction by releasing a variety of inhibitory cytokines and factors such as IL‐10, TGF‐β, and reactive oxygen species (ROS), as well as amino acid–degrading enzymes such as arginase 1 (Arg‐1) and indoleamine‐2,3‐dioxygenase (IDO). The gene discussed is ARG1; the disease is neoplasm.