CXCR4 (C-X-C Motif Chemokine Receptor 4) is a gene that encodes a receptor involved in the regulation of the immune system and cell movement. This receptor binds to its ligand, CXCL12, playing a critical role in guiding the migration and trafficking of immune cells. CXCR4 is vital for proper immune function and is associated with several diseases, including cancer and HIV infection.