Two members of the murine labial (lab) subfamily of Antennapedia-like homeobox-containing genes, Hox-1.6 and Hox-2.9, have been identified previously. Here we describe a third member genetically linked to the Hox-4 cluster on chromosome 2. This gene, designated Hox-4.9, is similar in structure to the other lab subfamily members. However, little coding sequence other than the homeobox and sequences immediately upstream of it have been conserved. By in situ hybridization analysis, Hox-4.9 mRNA is first detected at the end of the late streak stage (E7.75) in presumptive lateral and extraembryonic mesoderm. During early neurogenesis (E8.0-8.5), Hox-4.9 is detected solely in lateral mesoderm; its lack of expression in somitic mesoderm and the neural tube makes it unique among the Hox genes. By late neurogenesis and through mid-gestation (E9.0-E11.5), Hox-4.9 is no longer detected in lateral mesoderm but is found instead in a restricted region of presumed trunk neural crest and in the dermatome. These data are discussed in comparison with what is known about expression of the other members of the lab subfamily.