SMAD4 and familial dilated cardiomyopathy: qRT-PCR analysis demonstrated that the mRNA expression levels of Col1a1, Col3a1, TGF-β1, Smad2, Smad3, and Smad4 were considerably upregulated in DCM mice relative to controls, whereas all three exercise modalities downregulated their expression, and HIIT exerted the most pronounced inhibitory effect (Figure 4D–I).