Transforming growth factor (TGF)-1 is expressed in developing tooth from the initiation stage through adulthood. Odontoblast-specific expression of TGF-1 in the tooth continues throughout life; however, the precise biological functions of this growth factor in the odontoblasts are not clearly understood. Herein, we describe the generation of transgenic mice that overexpress active TGF-1 predominantly in the odontoblasts. Teeth of these mice show a significant reduction in the tooth mineralization, defective dentin formation, and a relatively high branching of dentinal tubules. Dentin extracellular matrix components such as type I and III collagens are increased and deposited abnormally in the dental pulp, similar to the hereditary human tooth disorders such as dentin dysplasia and dentinogenesis imperfecta. Calcium, one of the crucial inorganic components of mineralization, is also apparently increased in the transgenic mouse teeth. Most importantly, the expression of dentin sialophosphoprotein (dspp), a candidate gene implicated in dentinogenesis imperfecta II (MIM 125420), is significantly down-regulated in the transgenic teeth. Our results provide in vivo evidence suggesting that TGF-1 mediated expression of dspp is crucial for dentin mineralization. These findings also provide for the first time a direct experimental evidence indicating that decreased dspp gene expression along with the other cellular changes in odontoblasts may result in human hereditary dental disorders like dentinogenesis imperfecta II (MIM 125420) and dentin dysplasia (MIM 125400 and 125420).Mammalian development is a complex and highly orchestrated process that involves intricate cross-talk between growth factors and other regulatory molecules. These molecules interact with each other to induce specific molecular and cellular changes leading to organogenesis. Interactions between epithelium and mesenchyme are particularly crucial during the initiation of development of key organs such as teeth, skin, hair, mammary gland, and prostate (1). Tooth development is initiated by epithelial-mesenchymal interactions in the first branchial arch, and several transcription factors and growth factors are known to be expressed by dentin extracellular matrix (DECM)-producing 1 odontoblasts and enamel-producing ameloblasts during tooth development (2-5). Transforming growth factor-1 (TGF-1), a prototype of the TGF- superfamily, is a multi-functional growth factor expressed in a wide variety of developing tissues from the early stages. The regulation of cell proliferation, differentiation, embryonic development, and apoptosis by TGF-1 is well established (6 -8). During mouse tooth development, TGF-1 is expressed initially in the oral epithelium at embryonic day 13, and later its expression extends into the mesenchymal compartment and then gets restricted to the ectomesenchymal layer (odontoblasts). The odontoblast-restricted expression of TGF-1 persists throughout life in the mice (9). Odontoblasts produce DECM from embryonic day 16 and subseq...