Fibroblasts adjacent to mammary tumor cells cannot be differentiated into adipocytes because cytokines, such as tumor necrosis factor α (TNFα) and interleukins 6 and 11 (IL-6 and IL-11), secreted from malignant epithelial cells, suppress their differentiation, and also increase aromatase expression [14,46]. This evidence concerns the gene CYP19A1 and breast cancer.