As transgenic mice have become more commonly used as models for research in cardiovascular disease, a question arose as to whether the normal mouse heart responded to ischemia (lack of blood flow) the same way as hearts of other mammalian species. In addition, it is known that in most mammalian hearts, a phenomenon occurs whereby previous exposure to brief periods of ischemia can be protective against a prolonged ischemic episode, a phenomenon called ischemic preconditioning. Guo and colleagues* wondered whether mouse hearts exhibited preconditioning. Data from two of these groups, G, a control group, and a group preconditioned for 24 hours prior to an ischemic episode, are shown in Table D-36, Appendix D. Is there any evidence that ischemic preconditioning was effective, as evident by reduced infarct size, I, when accounting for the area at risk for infarction, R, as a covariate?