ERBB2 and breast cancer: RXR and partner receptors play important roles in the biology of macrophages and other immune cells.14 Because breast cancer progression is highly dependent on myeloid cell populations, specifically macrophages,2,5,6 we assessed immune cell populations (gating strategy for flow cytometry shown in Supplementary Fig. 1) and the state of activation of these populations in the tumors of MMTV-Neu mice.