As determined by western blotting with specific antibodies, exposure to ibrutinib led to a decrease in cyclin D1, E2F1, and phosphorylated Rb levels, and also a decrease in p-GSK3β levels (Fig. 1e); this suggested that inhibition of cyclin D1 and E2F1 expression, and of Rb and GSK3β phosphorylation, might play a role in ibrutinib-induced G1 arrest in GBM cells. This evidence concerns the gene RB1 and glioblastoma.