The Eutherian dental formula is described as 3/3 1/1 4/4 3/3. However, mice and rats have only one incisor and three molars, and there is a toothless area, diastema, between the incisor and first molar. It was demonstrated that primordia of the tooth germ is present in the diastema region, although these germs were arrested before growing into normal tooth. Gene expression of epithelial cells and mesenchymal cells in the diastema tooth primordia differed from that of the normal tooth germ. Since the mesenchymal cells did not condense around diastema primordial histologically, it is considerable that failure of cell condensation in the mesenchyme may lead to different gene expression in diastema tooth primordia compared with that of normal tooth germ.