Next we examined the site of cardiac iron deposition in these mice. 3,3′-Diaminobenzidine (DAB)-enhanced Perls staining showed marked ferric iron deposits within the cardiomyocytes of Fpn fl/fl Myh6.Cre+ mice, particularly at age 6 mo (Fig. 4D). Widespread deposition of electron-dense material (presumed to be iron) in lysosomes, around mitochondria, along sarcomeres, and within the dilated sarcoplasmic reticulum of Fpn fl/fl Myh6.Cre+ cardiomyocytes was also observed by electron microscopy (Fig. 4E).