ADRB2 and Left ventricular dilatation: While 25-week-old β2-60 (a dose of 60-fold overexpression of β2-AR) mouse had normal heart size and no phenotypic abnormalities, 25-week-old β2-350 (another dose of 350-fold overexpression of β2-AR) mouse had 4-chamber enlargement with severe left ventricular dilatation, atrial mural thrombi and aggressive or delayed cardiomyopathy14.