This study reports the structure of the mouse homeobox-containing gene Hox-1.4 of the HOX-1 cluster, as well as its expression pattern during embryonic and fetal development. The overall structure of this gene includes two major exons, the second of which encodes the homeo-domain. The putative Hox-1.4 protein displays similarities with products of homologous genes located at the same relative positions in other HOX clusters. A fragment extending 360 base pairs (bp) upstream of a transcriptional start site was shown to be able to promote transcription in transfected cells. This fragment is GC-rich and contains binding sites for the Spl transcription factor. In situ hybridization studies revealed the Hox-1.4 expression pattern during development. As already reported for several other murine Hox genes, Hox-1.4 is expressed in the fetal central nervous system (CNS), in structures derived from somitic mesodermal condensations (sclerotomes, prevertebrae) as well as in several mesodermal components of various organs and structures such as lungs, gut, stomach, intestine and meso- and metanephros. This expression pattern is in good agreement with recent proposals concerning the involvement of such genes in the establishment of the vertebrate body plan as well as the relationship between the positions of these genes within their clusters and the anteroposterior restriction of their expression domains.