In comparisons with our experiences with the other mouse models for acyl-CoA dehydrogenase deficiencies, the overall phenotype of MCAD−/− mice is less severe than that found in LCAD−/− mice, yet more pronounced than the VLCAD−/− or SCAD−/− mouse models [16,21,22]. All of these mutants are cold intolerant and display varying degrees of fatty changes in liver, heart, and kidney. LCAD−/− mice show more spontaneous deaths and gestational losses than the other deficiencies [21]. The significant neonatal mortality in MCAD−/− mice is distinctive from these other mouse models suggesting a greater degree of sensitivity to fasting intolerance. The phenotypes of both the VLCAD−/− and SCAD−/− mice are relatively mild if the animals are not cold stressed [16,22]. The MCAD-deficient mouse offers new insights into the pathogenesis of mitochondrial β-oxidation deficiencies and will provide a robust tool to better understand the role of fatty acids in other relevant diseases.