IDH1 and brain cancer: To date, alterations of four major pathways have been found in brain cancers, where at least one member of the signaling or metabolic circuit is mutated: RTK (receptor tyrosine kinases)/RAS (rat sarcoma)/PI3K (phosphotidylinositol-3-kinase) pathway, p53 pathway, RB pathway and the isocitrate dehydrogenase 1 or 2 (IDH1/2) pathway [7,8].