Additional file 1. Supplemental Figure 1. Characterization of iPSC#5 and iPSC#6. Supplemental Figure 2. Full length blots of Kir2.1 expression in untreated, Vector OE and KCNJ2 OE iPSC-CMs. Supplemental Figure 3. Expression of a panel of seven proteins in Vector OE and KCNJ2 OE iPSC-CMs. Supplemental Figure 4. Full length blots of Kir2.1 and Nav1.5 expression in Vector OE and KCNJ2 OE iPSC-CMs. Supplemental Figure 5. Full length blots of RYR2 and TNNT2 expression in Vector OE and KCNJ2 OE iPSC-CMs. Supplemental Figure 6. Full length blots of MYBPC3 and KCNH2 expression in Vector OE and KCNJ2 OE iPSC-CMs. Supplemental Figure 7. Full length blots of Cav1.2 and GAPDH expression in Vector OE and KCNJ2 OE iPSC-CMs. Supplemental Figure 8. Schematic representation of the approach to enhance maturation by overexpressing KCNJ2 in iPSC-CMs, which can provide more accurate prediction of drug-induced cardiotoxicity, and form more mature 3D human engineered heart tissues with better tissue structure and cell junction. Supplemental Table 1. Clinical features of six recruited patients. Supplemental Table 2. Summary of iPSC lines in this study. Supplemental Table 3. Primers used for qPCR in this study.