FAM20A has been studied to a very limited extent. Mutations in human FAM20 A cause amelogenesis imperfecta, gingival fibromatosis and kidney problems. It would be desirable to systemically analyse the expression of FAM20 A in dental tissues and to assess the pathological changes when this molecule is specifically nullified in individual tissues. Recently, we generated mice with a Fam20A-floxed allele containing the beta-galactosidase reporter gene. We analysed FAM20 A expression in dental tissues using X-Gal staining, immunohistochemistry and in situ hybridization, which showed that the ameloblasts in the mouse mandibular first molar began to express FAM20 A at 1 day after birth, and the reduced enamel epithelium in erupting molars expressed a significant level of FAM20 A. By breeding K14-Cre mice with Fam20Aflox/floxmice, we created K14-Cre;Fam20Aflox/flox(conditional knock out, c KO) mice, in which Fam20 A was inactivated in the epithelium. We analysed the dental tissues of c KO mice using X-ray radiography, histology and immunohistochemistry. The molar enamel matrix in c KO mice was much thinner than normal and was often separated from the dentinoenamel junction. The Fam20A-deficient ameloblasts were non-polarized and disorganized and were detached from the enamel matrix. The enamel abnormality in c KO mice was consistent with the diagnosis of amelogenesis imperfecta. The levels of enamelin and matrix metalloproteinase 20 were lower in the ameloblasts and enamel of c KO mice than the normal mice. The c KO mice had remarkable delays in the eruption of molars and hyperplasia of the gingival epithelium. The findings emphasize the essential roles of FAM20 A in the development of dental and oral tissues. FAM20A has been studied to a very limited extent. Mutations in human FAM20 A cause amelogenesis imperfecta, gingival fibromatosis and kidney problems. It would be desirable to systemically analyze the expression of FAM20 A in dental tissues and to assess the pathological changes when this molecule is specifically nullified in individual tissues. Recently, we generated mice with a Fam20A-floxed allele containing the beta-galactosidase reporter gene. We analyzed FAM20 A expression in dental tissues using X-Gal staining, immunohistochemistry and in situ hybridization, which showed that the ameloblasts in the mouse mandibular first molar began to express FAM20 A at 1 day after birth, and the reduced enamel epithelium in erupting molars expressed a significant level of FAM20 A. By breeding K14-Cre mice with Fam20A flox / floxmice, we created K14-Cre; Fam20Aflox / flox (conditional knock out, c KO) mice, in which Fam20 A was inactivated in the epithelium. We have analyzed the dental tissues of c KO mice using X-ray radiography, histology and immunohistochemistry. The molar enamel matrix in c KO mice was much thinner than normal and was often separated from the dentinoenamel junction. The Fam20A-deficient ameloblasts were non-polarized and disorganized and were detached from the enamel matrix The enamel abnormality in c KO mice was consistent with the diagnosis of amelogenesis imperfecta. The levels of enamelin and matrix metalloproteinase 20 were lower in the ameloblasts and enamel of c KO mice than the normal mice. The c KO mice had remarkable delays in the the eruption of molars and hyperplasia of the gingival epithelium. The findings emphasize the essential roles of FAM20 A in the development of dental and oral tissues.