GCGR and glioblastoma: We next used immunoblotting to specifically evaluate BTK protein expression in a panel of 29 GBM tumour tissue samples taken from 18 patients (patients 604, 376, 563, 1,285, 607 and 581 were sampled from multiple regions of each tumour) (Fig 5A), 10 GBM stem-like cell lines grown in serum-free media (GCGR-E20, GCGR-E21, GCGR-E22, GCGR-E25, GCGR-E27, GCGR-E28 GCGR-E34, GCGR-E56, GCGR-E64, and GCGR-E65) and one GBM cell line grown in serum-containing media (U251) (Fig 5B).