When animals died under observation, the heart was removed immediately and fixed for further histological examination. This examination showed significant enlargement of the left ventricular (LV) lumen and thinning of the LV wall in Fpn fl/fl Myh6.Cre+ hearts compared with Fpn fl/fl controls (Fig. 2B). These hearts also had altered cellular morphology, with dilated cardiomyocytes and elongated nuclei (Fig. 2B). By electron microscopy, hearts from Fpn fl/fl Myh6.Cre+ mice showed signs of myofibrillar disarray and an increased number of mitochondria in cardiomyocytes, not seen in hearts from Fpn fl/fl controls (Fig. 2C).