(A) Cardiac differentiation of human ESCs and iPSCs. Representative flow cytometry analysis at day 30 indicated that the total cell population consisted of ~85% cardiomyocytes as measured by cardiac troponin T (cTnT+) staining. (B) Fixed and stained cells were imaged using fluorescence microscopy and quantitatively analyzed. Scale bar 100 μM. (C) Comparable structures of ESC-CMs and iPSC-CMs in sarcomere length, cell perimeter, percentage of multinucleated cells and cell surface area (mean ± SEM; n = 30–40 cell per cell line). (D) Quantitative real-time PCR showed that calcium handling gene (CASQ2), gap junction (GJA5), potassium channel (KCNJ2, KCNJ5), structural genes (MYH6, MYH7), and sodium channel (SCN5A) were present in human ESC-CMs and iPSC-CMs at similar levels as those for human fetal heart tissue (mean ± SEM; n = 3 independent experiments). (E) Ca2+ transient of ESC-CMs and iPSC-CMs was measured using ratiometric dye Fura-2. Both ESC-CMs and iPSC-CMs displayed similar maturity in calcium handling (transient amplitude: ΔF/F0 = 3.8±0.3; time to peak: ~200 ms; transient duration: ~750 ms; 50% transient duration: ~400 ms; decay tau: ~250 ms) (mean ± SEM; n = 15–20 cells from 3 independent experiments).