Pallavi Sachdev and Fabien Décaillot showed that CXCR4 (chemokine CXC-type receptor 4) and CXCR7 form functional heterodimers in cell membranes.  The model show that in cells treated with the agonist ligand SDF-1, CXCR4 triggers inhibition of intracellular cAMP production and calcium mobilization, and both CXCR4 and CXCR7 stimulate ERK 1/2 activation (left panel). Co-expression of CXCR4 and CXCR7 leads to apparent heterodimerization of CXCR4 and CXCR7, and the heteromeric CXCR4-CXCR7 complex demonstrates a dramatically altered signaling profile. The CXCR4-CXCR7 heteromer is unable to trigger inhibition of cellular cAMP production.  The constitutive recruitment of b-arrestin2 couples CXCR4 stimulation in the CXCR4-CXCR7 complex to proliferative pathways (ERK1/2, p38 MAPK and SAPK).