Transcription factor specifically expressed in endothelial and hematopoietic precursor cells that acts as a key regulator of the endothelial differentiation cascade. Acts as an early-response transcription factor that regulates the expression of early regulators of endothelial and haematopoietic differentiation, such as etv2 and tal1. Heterodimer; efficient DNA binding requires dimerization with another bHLH protein. Specifically expressed in endothelial and hematopoietic precursor cells (PubMed:27411634). Expression peaks before the end of gastrulation and drops down by 24 hours post-fertilization, when the heart starts beating (PubMed:27411634). Expression precedes that of the earliest endothelial and haematopoietic markers etv2 and tal1 (PubMed:27411634). Loss of most endothelial and haematopoietic cells and a significant increase in cardiomyocyte numbers (PubMed:27411634, PubMed:7588049). Embryos show defects in the endothelial and hematopoietic lineages at a very early embryonic stage, characterized by the absence of the endocardium (PubMed:27411634). Early cardiac morphogenesis proceeds normally even in the absence of the endocardium (PubMed:27411634). The myocardial cells form a tube that is demarcated into chambers, beats rhythmically, but exhibits a reduced contractility (PubMed:27411634). Mutant embryos survive for one week of development (PubMed:27411634). Was named 'cloche', which means 'bell' in French, because of the bell-shaped heart in mutant embryos.