The advantage of the CXCR4-β-arrestin 2 reporter is that this system quantifies activation of CXCR4 in ovarian cancer cells by all sources of CXCL12 in the tumor microenvironment, while our previous assay detected only CXCL12 engineered with a fusion protein for complementation imaging. Here, CXCL12 is linked to ovarian carcinoma.