AbstractAlthough in mice, the dynamics of gene expression during heart development is well characterized, information on humans is scarce due to the limited availability of material. Here, we analyzed the transcriptional distribution of Mlc‐2a, Mlc‐1v, Mlc‐2v, and atrial natriuretic factor (ANF) in human embryonic hearts between 7 and 18 weeks of gestation and in healthy and hypertrophic adult hearts by in situ hybridization and compared expression with that in mice. Strikingly, Mlc‐2a, Mlc‐1v, and ANF, which are essentially chamber‐restricted in mice by mid‐gestation, showed a broader distribution in humans. On the other hand, Mlc‐2v may prove to be an adequate ventricular marker in humans in contrast to mouse. This study emphasizes the importance of careful comparative human–animal analyses during embryonic development and adulthood, as avoiding erroneous extrapolations may be critical to develop new and successful myocardial replacement therapies. Development Dynamics 235:1994–2002, 2006. © 2006 Wiley‐Liss, Inc.