NFI-C null mice demonstrate aberrant odontoblast differentiation and abnormal dentin formation, and thus develop molars lacking roots. However, other tissues and organs in the body including ameloblasts appear to be unaffected. Abnormal dentin in NFI-C null mice shares morphological similarities to the osteodentin that is formed in dental caries. However, little is known about the relationship between NFI-C and osteodentin formation. In this study, to elucidate the molecular characteristics of abnormal odontoblast in NFI-C null mice, we examined the levels of Ask-1, Cdc-2, Smad2/3, and TGF-betaR1 in cell culture and tissue sections from NFI-C null mice using immunofluorescence and immunohistochemistry. NFI-C protein was localized in the nucleus and cytoplasm of normal odontoblasts in vitro. Ask-1 and Cdc-2 proteins were shown in the perinuclear cytoplasm of both normal and NFI-C null mice. There were no differences in the pattern of production of Ask-1 and Cdc-2 proteins between normal and NFI-C null mice. Smad2/3 was not found in the odontoblast and subodontoblastic cells of the normal mice, whereas NFI-C null mice showed Smad2/3 immunoreactivity in the odontoblasts and subodontoblastic cells of the tooth pulp. TGF-betaR1 was weakly immunopositive in the odontoblast and subodontoblastic cells of normal mice, whereas it was detected strongly in the subodontoblastic cells of the NFI-C null mice. These results suggest that disruption of NFI-C increased the expression of Smad2/3 and TGF-betaR1 in developing odontoblasts and consequently caused abnormal dentin formation, similar to osteodentin.