Periodontal ligament (PDL) is a thin non-mineralized connective tissue between two mineralized tissues: alveolar bone and cementum. PDL keeps its function under enormous mechanical stress. In this article, I will introduce our studies characterizing the uniqueness of PDL and then discuss regeneration of periodontal tissue. S100A4 is a small Ca binding protein and we found high expression level of S100A4 in PDL. Histologically S100A4 localized on extracellular matrix and analysis of culture medium of PDL fibroblasts revealed secretion of S100A4 and recombinant S100A4 protein inhibited mineralization of osteoblastic culture. MC3T3-E1 cells (mouse osteoblastic cell line) expressed S100A4 at very low level and inhibition of S100A4 with a retroviral vector containing S100A4 antisense enhanced the mineralization of MC3T3-E1 cell culture. Finally, when PDL cells were exposed under mechanical stress in culture, expression level of S100A4 increased. These results indicate that S100A4 acts as a mineralization inhibitor in PDL and also bone; and that this protein plays a role in keeping its function under enormous mechanical stress. Although PDL is a non-mineralized tissue, it contains progenitors of osteoblasts and cementoblasts. Indeed, PDL is prerequisite for periodontal tissue regeneration. Application of guided tissue regeneration (GTR) and/or enamel matrix derivative (EMDOGAIN) promote periodontal tissue regeneration. Treatment with a dental implant, which is inserted to edentulous bone, is clinically acceptptable; however, a dental implant with PDL like a natural tooth would be more ideal.