STAT3 and central nervous system cancer: Down-signaling of self-activated glioma cells takes place via several pathways such as: phosphatase and tensin homolog deleted on chromosome ten/the mammalian target of rapamycin (PTEN/mTOR), signal transducer and activator of transcription 3/mitogen-activated protein kinase/extracellular signal-regulated kinases (STAT3/MEK/ERK) or Janus-activated kinases/signal transducers and activators of transcription (JAK/STAT), which promote PD-1L mRNA transcription, translation and presentation on the surface of cell [38,39].