Cholangiocarcinoma (CCA) is a malignancy of bile duct with the difficulty in early diagnosis, poor prognosis and less alternation in therapy. S100P is a member of S100 family proteins and plays important roles in cancers. We investigated the S100P expression and its correlation with clinicopathology in 78 cases of opisthorchiasis-associated CCA, and the effects of S100P knockdown with shRNA interference on the proliferation, cell cycle, migration, apoptosis and sensitivity to anti-cancer drug. Extremely high expression of S100P mRNA was detected in the CCA tumor tissues. The increased S100P protein expression was immunohistochemically confirmed and localized in the CCA cytoplasm and/or nuclei as well as in the hyperneoplasia and dysplasia bile ducts, but not in normal bile ducts. The intensity of immunostaining was correlated with survival, tumor stage and metastasis, and the high expression could be an independent prognostic factor. High levels of S100P were detected in the serum and bile fluid of CCA patients. The shRNA-mediated knockdown of S100P expression inhibited the proliferation in vitro and in vivo, and migration of CCA cells, arrested cell cycle with the up-regulated expression of cell cycle arrest related factors, p21, p27, GADD45A, and 14-3-3 zeta. S100P knockdown also promoted CCA cell apoptosis by up-regulating expression of apoptosis related factors, DR5, TRADD, caspase 3 and BAX, and increased the sensitivity of CCA cells to the chemotherapeutic agents sunitinib and apigenin. Taken together, this study indicates that S100P might be a promising biomarker for the diagnosis, prognosis and therapy of CCA.Cholangiocarcinoma (CCA) is a malignancy arising from bile ducts. Although it is rare, the incidence and mortality of the cancer have been increasing worldwide over the past decades, while the prognosis is remained dismal and substantially unchanged.1 Because of the lack of the method of early diagnosis, most of patients, when diagnosed, are incurable advanced and metastatic; their 5-year survival rate is extremely low. Chronic inflammation is one of important risks of CCA tumorigenesis, including Opisthorchis viverrini infection. Clear relationship between O. viverrini infection and CCA incidence has been demonstrated.2,3 The epidemiological investigations have indicated the astonished high incidence of CCA in opisthorchiasis endemic areas, especially in northeast of Thailand where both opisthorchiasis prevalence and CCA incidence are the highest in the world. 4,5 Serious O. viverrini infection, the lack of early diagnostic methods and poor prognosis of CCA put the millions of people at the risk of CCA in endemic areas. Although many efforts have been made, the biomarkers for diagnosis, prognosis and therapy of CCA are still quite limited. Therefore, it is urgent to delve into the tumorigenesis mechanism and develop novel biomarkers for the early diagnosis, prognosis, and therapy of the opisthorchiasis-associated CCA.Our previous cDNA microarray analysis showed that some of S100 proteins were up-re...