MPEG1 and breast cancer: Therefore, to test whether the host immune system is responsible for the observed reduction in injected tumor cells over time, we investigated the role of immune cells, specifically macrophages, by injecting metastatic MDA-MB231_eGFP (breast cancer) cells into the Tg(mpeg1:mCherry) zebrafish reporter line, in which macrophages are labeled by mCherry38.