Receptor component of the CCM signaling pathway which is a crucial regulator of heart and vessel formation and integrity. May be acting by stabilizing endothelial cell junctions. Interacts with CCM2 and KRIT1; KRIT1 markedly facilitates interaction with CCM2. Expressed in the endothelium of the developing heart and aorta and in the neural tube at 10.5 dpc, and in the arterial endothelium, smooth muscle, endocardium of the heart and brain vasculature at 14.5 dpc. Some mutant animals die in utero, but most die postnatally, about half before weaning as a result of pulmonary hemorrhage. Midgestation mutant embryos show cardiac defects characterized by invagination of the ventricular cavity into, and often through, the compact layer of ventricular myocardium. The septal myocardium is similarly honeycombed by endothelial-lined extensions from the ventricular cavity, a defect accompanied by the presence of ventricular septal defects in most late-gestation embryos. Neonatal mice also show defective cardiac integrity manifested by a blood-filled pericardial sac, due to the rupture of the low-pressure atrial chamber of the heart. Cardiac or pulmonary integrity defects are observed in half of the animals. About 10% of neonatal mutant mice exhibit dilated lymphatic vessel malformations. May be due to an intron retention. Truncated N-terminus.