Several class A basic helix-loop-helix (bHLH) transcription factors have been cloned from the developing mouse and chick nervous system. The cloned cDNAs (ME1, ME2, ME3, ME4, in the mouse and GE1, GE2 in the chick) have HLH coding regions highly homologous to other known class A bHLH genes. The genes corresponding to ME1 and GE1 are abundantly expressed during development of the central nervous system. ME1 and GE1 are expressed in proliferating neuroblasts and in cells at the initial stages of differentiation (for example in the external granule cell layer of the cerebellum and in the lateral region of the ventricular zone in the developing neural tube and cortex). They are also expressed at high levels in morphogenetically active regions such as limb buds, somites and mesonephric tubules. The expression of ME1 and GE1 decreases once cellular differentiation is over. Based on the expression of ME1 and GE1 in regions of active cellular proliferation and differentiation and on the known role of other bHLH factors in development, we suggest that ME1 and GE1 play important roles during development of the nervous system as well as in other organ systems.