GPC1 and glioblastoma: Qiao et al. demonstrated ectopic expression of GPC1 stimulates S phase entry via downregulation of tumor suppressors, including pRb and Cip/Kip cyclin-dependent kinase inhibitors, and upregulation of pro-oncogenic proteins, including cyclin E and cyclin-dependent kinase 2, in the human glioblastoma cell line U87-MG [19].