CD44 and cancer: The heterogeneous expression of HER family members and putative cancer stem cell biomarkers suggests the complex regulatory network that may drive brain tumour progression and its resistance to therapy with various types of EGFR inhibitors [14,49,116,117,118] Therefore, further investigations involving a larger group of patients are warranted to validate the relative expression, co-expression, and prognostic significance of all members of the HER family with EGFRvIII and CD44 in patients with brain tumours.