P2RY6 and central nervous system cancer: There is ample evidence that these protein kinases are involved in the proliferative phenomenon activated by G protein-coupled receptors in various cell systems [e.g., [27,28]]; in addition, p44 and p42 MAPK activation dependent on P2Y2 or P2Y6 receptors has been described, e.g., in granulosa-luteal cells [14], glioma cells [35], and embryonic stem cells [36].