Creation of IDKO mice. (A) A Western blot using nuclear extracts prepared from ventricles of the indicated genotypes and demonstrating reductions in Rb protein levels after TAM treatment. (B) Expression of fetal cardiac-specific genes is up-regulated specifically in IDKO hearts. Semiquantitative RT-PCR was performed on total RNA extracted from purified ACMs with the indicated genotypes. For quantitation of changes in gene expression, see Fig. S3 A. CaA, cardiac actin; SkA, skeletal actin. (C) Hematoxylin and eosin staining of myocardial sections from mice with the indicated genotypes 1 wk after TAM treatment. Bars, 40 µm.