Lactic acid produced by tumour cells was reported earlier to polarise macrophages into an M2-like phenotype, with Arg1 expression by macrophages essential for lung cancer and melanoma growth.15 In addition, Carmona-Fontaine et al. have demonstrated that lactate cooperates with hypoxia to induce the expression of ARG1 in macrophages. The gene discussed is ARG1; the disease is lung cancer.