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Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) are a valuable tool to characterize the pharmacology and toxic effects of drugs on heart cells. In particular, hiPSC-CMs can be used to identify drugs that generate arrhythmias. However, it is unclear whether the expression of genes related to generation of CM action potentials differs between hiPSC-CM cell lines and the mature human heart. To address this, we obtained accurate gene expression profiles of commercially available hiPSC-CM cell lines with quantitative real time RT-PCR analysis. Expression analysis of ten cardiac proteins important for generation of action potentials and three cardiac proteins important for muscle contractility was performed using GAPDH for normalization. Comparison revealed large variations in expression levels among hiPSC-CM cell lines and between hiPSC-CMs and normal human heart. In general, gene expression in hiPSC-CM cell lines was more similar to an immature, stem-like cell than a mature cardiomyocyte from human heart samples. These results provide quantitative information about differences in gene expression between hiPSC-CM cell lines, essential for interpreting pharmacology experiments. Our approach can be used as an experimental guideline for future research on gene expression in hiPSC-CMs. Keywords: iPS cells, Quantitative real time-PCR, Reference gene, Cardiac ion channels, Action potential

3-Hydroxyisobutyryl-CoA hydrolase (HIBCH) deficiency is an autosomal recessive disorder characterized by episodes of ketoacidosis and a Leigh-like basal ganglia disease, without high concentrations of pyruvate and lactate in the cerebrospinal fluid. Only 4 cases of HIBCH deficiency have been reported. However, clinical–biochemical correlation in HIBCH deficiency by determining the detailed residual enzyme activities has not yet been elucidated. Here, we report a case of two Japanese siblings with HIBCH deficiency carrying a new homozygous missense mutation (c.287C > A, [p.A96D]) at the substrate-binding site. A transfection study using HIBCH expression vectors harboring wild type or 4 reported mutations, including the newly identified mutation (p.A96D, p.Y122C, p.G317E, and p.K74Lfs*13), revealed a correlation between residual HIBCH activities and the severity of the disease. All HIBCH mutants, except p.K74Lfs*13, showed residual enzyme activity and only the patient with p.K74Lfs*13 had congenital anomalies. p.G317E showed only low enzyme activity (~3%) of that of wild-type HIBCH. Although p.A96D had approximately 7 times higher enzyme activity than p.G317E, patients with p.A96D died during childhood. These findings are essential for clinical management, genetic counseling, and specific meal and concomitant drug considerations as part of the treatment for patients with HIBCH deficiency.