Hereditary multiple exostoses (EXT) is a genetically heterogeneous, autosomal dominant skeletal disorder. The gene for EXT1 maps to human chromosome 8q24.1 and encodes an evolutionary conserved protein that is a member of a multigene family. The mouse homolog of human EXT1 protein is 99% similar to its human counterpart. Here, we present the expression profiles of the mouse EXT1 gene. EXT1 mRNA is initially expressed at 6.5 days post-coitum (d.p.c.), which coincides with gastrulation of the mouse embryo. Whole mount in situ hybridization with 10.5 to 12.5 d.p.c. mouse embryos showed a high level of expression of EXT1 mRNA in developing limb buds. Epitope tagging experiments revealed the endoplasmic reticulum localization of EXT1 protein. This localization was consistent with a hydrophobic stretch of amino acids present at the N-terminal end of the EXT1 protein. These results provide novel information on the function of EXT1 and the etiology of hereditary multiple exostoses.