As Pparγ did not increase the proportion of F4/80+ macrophages in the bone marrow, blood or spleen, these studies suggest the reduction in tumor-associated F4/80+ macrophages within the ErbB2 Pparγ1−/− mammary tumors is not due to a Pparγ1-mediated alteration in hematopoietic precursors in the bone marrow or the circulation. The gene discussed is ERBB2; the disease is breast cancer.