A study by Zhou et al. [20] has suggested that BMDM are the main source of macrophages in GBM and that their recruitment from the bloodstream is stimulated by GBM-secreted periostin (POSTN) that interacts with integrin αvβ3 (ITGαvβ3) on BMDMs (Figure 1, Molecular event 2). This evidence concerns the gene POSTN and glioblastoma.