CXCR4 and heart failure: CXCR4 cKO mice exhibited significant declines in both contractility and ejection fraction: (1) A 2.5× decrease (** p < 0.01) in contractility, as indicated by the maximum rate of change in left ventricular pressure (dp/dtmax); and (2) a significant decrease (* p < 0.05) in ejection fraction as compared to littermate controls, indicating mild, but progressive heart failure (Figure 3B).