MESP1, also known as bHLHc5, is a 268 amino acid protein that contains one basic helix-loop-helix (bHLH) domain, a motif that mediates protein dimerization and can bind to the E-box sequence of DNA. Localized to the nucleus, MESP1 functions as a transcription factor that, via its bHLH domain, participates in the epithelialization and the development of the cardiac and somitic mesoderm. MESP1 is highly expressed during gastrulation and somitogenesis and is necessary for the formation of single heart tubes during cardiac maturation. Early detection of MESP1 may be an indicator of the formation of cardiac precursor cells in developing embryos. Additionally, MESP1 plays a role in the rostrocaudal patterning of the somites, an event that influences select Notch signaling pathways.