Targeted deletion of Runx genes has revealed distinct roles for these proteins in development. Here, we report that epithelial Runx genes are involved in regulation of ameloblastdifferentiation. We analyzed the functional role of Cbfb and Runx1 in the enamel formation invivo, using a conditional genetic approach in which the Cbfb and Runx1 gene was inactivated in mouse epithelial cells(K14-Cre/Cbfbfl/fl, K14-Cre/Runx1 fl/fl). We found that mice deficient in epithelial Cbfb and Runx1 resulted in enamel aplasia in the incisors and in hypoplasia in the molars. The present results provide the genetic evidence that Runx1 and Cbfb genes function in the enamel formation in developing molar and incisors.