['Matricellular proteins, including periostin, modulate cell-matrix interactions and cell functions by acting outside of cells.', 'In this study, however, we reported that periostin physically associates with the Notch1 precursor at its EGF repeats in the inside of cells. Moreover, by using the periodontal ligament of molar from periostin-deficient adult mice (Pn-/- molar PDL), which is a constitutively mechanically stressed tissue, we found that periostin maintained the site-1 cleaved 120-kDa transmembrane domain of Notch1 (N1) level without regulating Notch1 mRNA expression. N1 maintenance in vitro was also observed under such a stress condition as heat and H(2)O(2) treatment in periostin overexpressed cells. Furthermore, we found that the expression of a downstream effector of Notch signaling, Bcl-xL was decreased in the Pn-/- molar PDL, and in the molar movement, cell death was enhanced in the pressure side of Pn-/- molar PDL.'] Does periostin associate with Notch1 precursor to maintain Notch1 expression under a stress condition in mouse cells?