TNF and neoplasm: Brain metastatic tumour cells can transfer the second messenger cGAMP to astrocytes via establishing carcinoma-astrocyte gap junctions, activating the STING pathway and production of inflammatory cytokines such as IFN-α and TNF, which activate the STAT1 and NF-κB signalling in brain metastatic cells, thereby supporting tumour growth and chemoresistance [240].