EGFR and central nervous system cancer: The rapid and sustained inhibition of ERK phosphorylation observed in response to erlotinib could be used as a biomarker of appropriate drug delivery in vivo (i.e. to test whether the drug has reached the target), as opposed to a biomarker of drug efficacy, which fits clearly better with the monitoring of Akt phosphorylation, in agreement with the reported correlation between EGFR inhibitor sensitivity and Akt inhibition found in glioma cell lines [25] and glioma tumour-initiating cells (TICs) [28].