NTRK1 and glioblastoma: Receptor tyrosine kinase activation (“RTK”) index, based on the levels of Tyr phosphorylation corresponding to activation of 11 RTKs (S1 Table), decreased in 2% FBS (SDCs) relative to CSCs in HF2927 and HF3016, the only 2 models harboring EGFR amplification on extrachromosomal DNA (ecDNA) [11], and increased in the remaining 6 GBM models (Fig 3A).