Protocadherins and Connexin-43 (Cx43)-mediated gap junctions, where the transfer of the second messenger cytosolic guanosine–adenosine monophosphate (cGAMP) activates the STING pathway in astrocytes and instructs them to produce and secrete tumor-stimulating cytokines (e.g., INF-α, TNF-α) are thought to be the means by which tumor cells from brain metastasis communicate with local astrocytes. Here, GJA1 is linked to neoplasm.