ARRB2 and hydrops fetalis: More specifically, we found that cardiac βarr1 (and not βarr2) is the βarr isoform responsible for the aforementioned βAR desensitization and downregulation (Figure 1), the molecular hallmark of HF, since βarr1KO mice have significantly better cardiac β-adrenergic and inotropic reserves, accompanied by elevated cardiac functional βAR numbers and cAMP pro-contractile signaling, compared to control, age-matched wild type mice, both in healthy conditions and, even more so, after MI.