Our findings showing a lack of cardiomyocyte PH3 staining in both the RV and LV in HLHS, and in the RV in TF, but easily detectable evidence of proliferation in stromal cells, confirms the findings in previous studies indicating that mesenchymal and endothelial cells, but not cardiomyocytes, are highly proliferative throughout life [27]. The gene discussed is TF; the disease is hypoplastic left heart syndrome.