Our analysis uncovered robust differential methylation cg03800765 in both iPSC-CMs (– 32.4%, P = 9.0 × 10–6) and cardiac biopsies (– 25.2%, P = 0.004), a CpG site located within a promoter-associated CpG island upstream of ATG7. Although methylation at this site was negatively correlated with ATG7 expression (P = 0.0026), only cg03800765 methylation was significantly predictive of patient diagnosis with HF in iPSC-CMs (P < 0.0001), cardiac biopsies (P = 0.0167), and circulating cells (P < 0.0001); by contrast, ATG7 expression failed to provide any diagnostic benefit (P = 0.264). The gene discussed is ATG7; the disease is hydrops fetalis.