Mouse embryonic stem (ES) cells, when cultivated as embryo-like aggregates, are able to differentiate in vitro into derivatives of all primary germ layers. These include functionally active cardiomyocytes representing ventricle-like, atrium-like, and pacemaker-like cells. During differentiation, a developmentally controlled expression pattern of cardiac- specific genes, proteins, action potentials, ion channels, and receptors is found. This developmental pattern can be modulated in vitro by differentiation factors such as retinoic acid or by 'gain-of-function' and 'loss-of-function' approaches. The latter strategy was successfully used for the characterization of cardiac phenotypes of B₁ integrin-deficient ES cells after differentiation into the cardiogenic lineage.