CXCR4 is a receptor for the C-X-C chemokine CXCL12/SDF-1. It is involved in hematopoiesis and in cardiac ventricular septum formation, and it also plays an essential role in vascularization of the gastrointestinal tract by regulating vascular branching and remodeling processes in endothelial cells. Furthermore, it is involved in cerebellar development, and in the CNS it may function to mediate hippocampal-neuron survival. During embryogenesis and adult life, CXCR4 is important for neuronal guidance and mutations in the gene result in abnormal neuronal distribution. CXCR4 is also a co-receptor (CD4 being the primary receptor) for HIV-1 X4 isolates and a primary receptor for some HIV-2 isolates, and it promotes Env-mediated fusion of the virus. In immunohistochemistry, CXCR4 has membranous positivity in immune cells and the adrenal gland, with some expression in the lung, gastrointestinal tract and other tissues. References: FEBS Letters. 426 (2): 271–8, PMID: 9599023; Journal of Theoretical and Computational Chemistry. 2019. 1 (1): 1–9, DOI: 10.1142/S0219633619500184; Development. 2002. 129 (18): 4249–60, PMID: 12183377.