MASH1 and MASH2 (Mammalian achaete-schute Homolog 1 and 2) are basic helixloop-helix transcription factors which are mammalian homologs of the achaete-schute gene family that is required for neuronal development in Drosophila. The bHLH motif is present in many types of transcription factors including E2-2, E47, MyoD and others. MASH1 has been shown to heterodimerize with other bHLH containing transcription factors. MASH1-E12 heteromers bind to and promote transcriptional activation of muscle creatine kinase, a known target for MyoD activation. However, MASH1 appears to play its primary role during early development of the autonomic nervous system. In committed neuronal precursor cells, early expression of MASH1 activates a subset of neuron-specific genes to promote differentiation. Thus null mutations in the MASH1 gene suggest that MASH1 is a valuable marker for investigation of neural development in mammals. MASH1 is observed as an ~34 kDa protein by SDS-PAGE.