The Heart of glass-Cerebral Cavernous Malformation (Heg-CCM) pathway is essential for heart development in zebrafish and mouse. In zebrafish, mutants for the Heg-CCM genes ccm1, ccm2, and heg exhibit an extreme dilation of the heart chambers and inflow tract and completely lack blood circulation. The mechanisms by which this pathway regulates heart development are incompletely understood. Two major impediments to our knowledge are the paucity of genes known to participate in the Heg-CCM pathway and a lack of information about how the Heg-CCM pathway interacts with other signaling pathways in live embryos.