As shown in Figure 5A, purified IgGs (0.1 μmol/L) from HF patient sera positive for β3-AR Abs markedly decreased cardiomyocyte beating frequency from 93.56±5.47/min to 64.32±8.13/min after 1 hour of exposure, suggesting a negative chronotropic effect of β3-AR Abs. This evidence concerns the gene ADRB3 and hydrops fetalis.