The Nkx2.6 gene belongs to the NK superfamily of homeobox genes (Harvey, 1996). We report here the expression pattern of the murine Nkx2.6 gene during early mouse development, which is unique among the NK family of homeobox genes in that its expression is restricted to the very narrow development period between stages E8.5 and E10.5 of embryogenesis. The distribution of Nkx2.6 transcripts is also quite restricted spatially, with expression detected uniquely within the caudal branchial arches. Nkx2.6 is expressed in all three layers comprising the caudal branchial arches (ectoderm, mesectoderm and endoderm) with the strongest expression being detected in the surface ectoderm.