TLR9 and posterior cortical atrophy: Furthermore, ligands and their receptors, such as CXCL12 and CXCR4 (20), tumor-associated exosomes (21), transmembrane proteins, such as CXCR6 (22) and TLR9 (23), protein kinases and related signaling pathways, have already been found to be involved in PCa metastasis.