For example, in breast cancer, tumor-secreted miR-122 reprograms glucose metabolism in the pre-metastatic niche by inhibiting the glycolytic enzyme pyruvate kinase M2 (PKM2) [29], while in brain cancer, astrocyte-derived miR-19a secreted by the niche cells induces the loss of PTEN in tumor cells [30]. This evidence concerns the gene PTEN and brain cancer.