In mammary cancer cells, IL-6 decreased proliferation via regulation of matrix metalloproteinases [212], thus IL-6 may directly regulate the tumor microenvironment; it also exhibits anti-tumorigenicity by increasing natural killer cell activity and intra-tumoral infiltration; these lymphocyte-mediated responses are necessary, but not sufficient, for anti-tumorigenicity [161], suggesting that other exercise-related humoral or additional factors are also involved. This evidence concerns the gene IL6 and breast cancer.