In a study on brain cancer, human and mouse tumor cells were observed to stop expressing phosphatase and tensin homolog (PTEN)—an important tumor suppressor—after dissemination to the brain due to inhibition mediated by exosomal miR-19 that was secreted by astrocytes, which indicates that exosomal miR-19 might be a suitable biomarker for diagnosing brain cancer metastasis [66]. This evidence concerns the gene PTEN and brain cancer.