The CXCR4 (C-X-C chemokine receptor type 4) protein is one member of the G-protein coupled receptor (GPCR1) family. As a multipass membrane protein that is found in several tissues, it is the receptor for the C-X-C chemokine CXCL12/SDF-1. The CXCR4 ligand works by modulating intracellular calcium ion levels and activating the MAPK1/MAPK3 signal pathways. CXCR4 is also a receptor for extracellular ubiquitin, which produces enhanced levels of intracellular calcium and reduced levels of cellular cAMP.