Whole mount in situ hybridisation was used to study the embryonic expression of the mouse HMG box-containing genes Sox1, Sox2 and Sox3 between 6.5 and 9.0 days post coitum (dpc). Sox2 and Sox3 are expressed in the epiblast and extraembryonic ectoderm of the egg cylinder, becoming restricted to the prospective neural plate and chorion at the onset of gastrulation. Sox3 is upregulated in the posterior ectoderm during late streak and neural plate stages and is concomitantly downregulated in the chorion. Sox1 transcripts are first detected in the neural fold ectoderm at the headfold stage. During early somitogenesis, all three genes are expressed in the neuroectoderm, and Sox2 and Sox3 are also expressed in the primitive streak ectoderm, gut endoderm and prospective sensory placodes.