CD74-positive microglia, the resident macrophage-like cells of the central nervous system (CNS), promote the growth of primary brain tumors through suppression of the antitumor immune response [29], and STAT3+ reactive astrocytes associated with brain metastases showed increased expression of the CD74 ligand MIF (macrophage migration inhibitory factor) and increased binding to CD74+ microglia (Fig. 1a(b)). This evidence concerns the gene STAT3 and brain neoplasm.