Using a series of genetic and pharmacological, gain- and loss-of-function, in vitro and in vivo approaches, here we uncover macrophage PPARγ as an important suppressor of breast cancer progression and a key mediator of the anti-tumor effects of rosiglitazone that functions by repressing a novel target Gpr132 in macrophages. The gene discussed is GPR132; the disease is breast carcinoma.