Transcription factor that plays a key role in neuronal differentiation: acts as a pioneer transcription factor, accessing closed chromatin to allow other factors to bind and activate neural pathways (PubMed:24243019). Directly binds the E box motif (5'-CANNTG-3') on promoters and promotes transcription of neuronal genes (PubMed:20107439, PubMed:24243019, PubMed:27281220). The combination of three transcription factors, ASCL1, POU3F2/BRN2 and MYT1L, is sufficient to reprogram fibroblasts and other somatic cells into induced neuronal (iN) cells in vitro (PubMed:20107439, PubMed:24243019, PubMed:27281220). Plays a role at early stages of development of specific neural lineages in most regions of the CNS, and of several lineages in the PNS (PubMed:8217843). Essential for the generation of olfactory and autonomic neurons (PubMed:8221886). Acts synergistically with FOXN4 to specify the identity of V2b neurons rather than V2a from bipotential p2 progenitors during spinal cord neurogenesis, probably through DLL4-NOTCH signaling activation (PubMed:16020526, PubMed:17728344). Involved in the regulation of neuroendocrine cell development in the glandular stomach (PubMed:18173746). Efficient DNA binding requires dimerization with another bHLH protein. Forms a heterodimer with TCF3. Developing CNS and PNS at embryonic and postnatal stages. Expressed in the epithelium of glandular stomach (PubMed:18173746). Between 8.5 dpc and 10.5 dpc it is found in the neuroepithelium of the midbrain and ventral forebrain, as well as in the spinal cord. Between 10.5 dpc and 12.5 dpc its expression pattern changes from a restricted to a widespread zone, it is then found at variable levels in the ventricular zone in all regions of the brain, where is expressed in a subset of p2 progenitors that can give rise to either V2a or V2b interneuron subtypes. From 12.5 dpc to postnatal stages it is also expressed in cells outside of the ventricular zone through the brain, and in addition it is also expressed during development of the olfactory epithelium and neural retina. At 12.5 dpc, it is highly expressed by differentiating enteric neurons in the mesenchyme of the stomach. At 14.5 and 16.5 dpc, it is also expressed in the epithelium of the glandular stomach (PubMed:18173746). Lethality at birth caused by severe defects in neurogenesis. While the brain and spinal cord of the mutants appear normal, their olfactory epithelium and sympathetic, parasympathetic and enteric ganglia are severely affected. In the olfactory epithelium, neuronal progenitors die at an early stage, whereas the non-neuronal supporting cells are present. In sympathetic ganglia, the development of neuronal precursors is arrested, preventing the generation of sympathetic neurons, without affecting glial precursor cells. Homozygous MASH1-null mice have smaller stomachs than the control, and neuroendocrine cells are mostly missing, while chief, parietal and pit cells are formed. However, the wall of the glandular stomach is much thicker, has a deeper fold structure, and the forestomach epithelium is villous compared to controls (PubMed:18173746).