A method of diagnosing and treating a human glioblastoma multiforme (GBM) brain tumor in a subject is disclosed. The method includes administering to the subject, an effective amount of composition having a peptide 12-20 amino acid residues in length and selected for its ability to bind preferentially to a subtype of human GBM cells identified as brain tumor initiating cells (BTICs) or highly invasive glioma cells (HIGCs).