Macrophages enhanced tumor growth of humanized glands. Murine mammary glands were humanized +/- the addition of primary human macrophages and exogenous estrogen supplementation (5 mice per treatment group). Two weeks post-humanization, 1 × 106 T47D breast cancer cells were injected with fibroblasts or fibroblasts and macrophages into the humanized region. One tumor out of a total of five injected mice developed tumors in the standard humanized treatment group, while three tumors out of five injected mice developed tumors in the glands humanized with macrophages. Data represent mean +/- SD.