Currently, a variety of indicators are used to assess the response to anti-angiogenic therapies in recurrent glioblastoma, including non-invasive diagnostic biomarkers such as phosphatidylinositol-glycan biosynthesis class F (PIGF) [44], interleukin-8 (IL-8) [45] and circulating collagen IV [46]. Here, PIGF is linked to glioblastoma.