Nhlh1 is a basic helix-loop-helix (bHLH) gene that has been implicated in mouse neurogenesis. Previous studies have shown it to be expressed in regions in which there are differentiating neurons during late embryonic and fetal development, but detailed studies of the role of Nhlh1 earlier in embryonic development have not been performed. In this paper, we examine the expression of Nhlh1 transcripts at early embryonic stages (E8.5-E10.5), at the onset of neurogenesis, and compare the pattern of expression with that of Islet-1, a marker of postmitotic neurons. We show that Nhlh1 is expressed in early postmitotic neurons but is down-regulated as these cells migrate from the ventricular zone. We have also determined the genomic structure of mouse Nhlh1 and have characterised the promoter sequence, as a first step towards identifying factors that may control Nhlh1 expression. Nhlh1 has been implicated previously as a candidate for the neural tube defect mutant loop-tail (Lp); here, we present sequence and expression data indicating that Nhlh1 is unlikely to be responsible for the Lp mutation.