CM2 expressed high levels of maladaptive markers for the reactivation of the fetal gene program such as NPPB (encoding natriuretic peptide B, a clinical biomarker for heart failure) and ACTA1 (encoding skeletal α-actin)12, which thus denotes a subpopulation of cardiomyocytes towards a failing state (Fig. 2b). Here, NPPB is linked to heart failure.