Transcriptional factor that may have pleiotropic effects during embryogenesis and in the adult. Efficient DNA binding requires dimerization with another bHLH protein. Heterodimer; forms a heterodimer with ARNT, ARNT2. Detected in lung, skeletal muscle and kidney. During fetal development it is found in the CNS, developing kidney, mesodermal and endodermal tissues, including developing somites, mesonephric duct, and foregut. Truncated C-terminus.