ACSL1, alternatively titled Hash1 or Mash1, is a member of the basic helix-loop-helix (BHLH) family of transcription factors. It activates transcription by binding to the E box (5'-CANNTG-3'). Dimerization with other BHLH proteins is required for efficient DNA binding. ACSL1 plays a role in the neuronal commitment and differentiation and in the generation of olfactory and autonomic neurons. The protein is highly expressed in medullary thyroid cancer and small cell lung cancer and may be a useful marker for these cancers. The presence of a CAG repeat in the gene suggests it may also play a role in tumor formation.