CXCR4 and neoplasm: In order to investigate the mechanism responsible for the lack of significant differences between these two models, we used a PET tracer developed by our group, [18F]MCFB; this tracer is a fluorine-18 labelled small molecule analogue of the CXCR4-targeting AMD3465, that due to its small size can extravasate the tumor vasculature and penetrate all tumor compartments, providing readouts of whole-tumor CXCR4 expression.