FZD4 and neoplasm: Several GPCR ligands and accessory proteins have been shown to contribute to tumor vascularization, including lysophosphatidic acid (LPA), sphingosine 1P, thrombin, angiotensin, prostaglandin, melatonin and diverse interleukin and chemokines [22,23], suggesting that GPCR-mediated signaling may support vessel formation in diverse tumor types (Figure 2).