Background::Gene promoter methylation is a major epigenetic change in cancers, which plays critical roles in carcinogenesis. As a crucial regulator in the early stages of B-cell differentiation and embryonic neurodevelopment, the paired box 5 (n PAX5) gene is downregulated by methylation in several kinds of tumors and the role of this downregulation in esophageal squamous cell carcinoma (ESCC) pathogenesis remains unclear.n Methods::To elucidate the role of n PAX5 in ESCC, eight ESCC cell lines, 51 primary ESCC tissue samples, and eight normal esophageal mucosa samples were studied and The Cancer Genome Atlas (TCGA) was queried. n PAX5 expression was examined by reverse transcription-polymerase chain reaction and western blotting. Cell apoptosis, proliferation, and chemosensitivity were detected by flow cytometry, colony formation assays, and 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide assays in ESCC cell lines with n PAX5 overexpression or silencing. Tumor xenograft models were established for n in vivo verification.n Results::PAX5 methylation was found in 37.3% (19/51) of primary ESCC samples, which was significantly associated with age (n P = 0.007) and tumor-node-metastasis stage (n P = 0.014). TCGA data analysis indicated that n PAX5 expression was inversely correlated with promoter region methylation (n r = -0.189, n P = 0.011 for cg00464519 and n r = -0.228, n P = 0.002 for cg02538199). Restoration of n PAX5 expression suppressed cell proliferation, promoted apoptosis, and inhibited tumor growth of ESCC cell lines, which was verified in xenografted mice. Ectopic n PAX5 expression significantly increased p53 reporter luciferase activity and increased p53 messenger RNA and protein levels. A direct interaction of n PAX5 with the p53 promoter region was confirmed by chromatin immunoprecipitation assays. Re-expression of n PAX5 sensitized ESCC cell lines KYSE150 and KYSE30 to fluorouracil and docetaxel. Silencing of n PAX5 induced resistance of KYSE450 cells to these drugs.n Conclusions::As a tumor suppressor gene regulated by promoter region methylation in human ESCC, n PAX5 inhibits proliferation, promotes apoptosis, and induces activation of p53 signaling. n PAX5 may serve as a chemosensitive marker of ESCC.n “,”Background::Gene promoter methylation is a major epigenetic change in cancers, which plays critical roles in carcinogenesis. As a crucial regulator in the early stages of B-cell differentiation and embryonic neurodevelopment, the paired box 5 (n PAX5) gene is downregulated by methylation in several kinds of tumors and the role of this downregulation in esophageal squamous cell carcinoma (ESCC) pathogenesis remains unclear.n Methods::To elucidate the role of n PAX5 in ESCC, eight ESCC cell lines, 51 primary ESCC tissue samples, and eight normal esophageal mucosa samples were studied and The Cancer Genome Atlas (TCGA) was queried. n PAX5 expression was examined by reverse transcription-polymerase chain reaction and western blotting. Cell apoptosis, proliferation, and chemosensitivity were detected by flow cytometry, colony formation assays, and 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2-H-tetrazolium bromide assays in ESCC cell lines with n PAX5 overexpression or silencing. Tumor xenograft models were established for n in vivo verification.n Results::PAX5 methylation was found in 37.3% (19/51) of primary ESCC samples, which was significantly associated with age (n P = 0.007) and tumor-node-metastasis stage (n P = 0.014). TCGA data analysis indicated that n PAX5 expression was inversely correlated with promoter region methylation (n r = -0.189, n P = 0.011 for cg00464519 and n r = -0.228, n P = 0.002 for cg02538199). Restoration of n PAX5 expression suppressed cell proliferation, promoted apoptosis, and inhibited tumor growth of ESCC cell lines, which was verified in xenografted mice. Ectopic n PAX5 expression significantly increased p53 reporter luciferase activity and increased p53 messenger RNA and protein levels. A direct interaction of n PAX5 with the p53 promoter region was confirmed by chromatin immunoprecipitation assays. Re-expression of n PAX5 sensitized ESCC cell lines KYSE150 and KYSE30 to fluorouracil and docetaxel. Silencing of n PAX5 induced resistance of KYSE450 cells to these drugs.n Conclusions::As a tumor suppressor gene regulated by promoter region methylation in human ESCC, n PAX5 inhibits proliferation, promotes apoptosis, and induces activation of p53 signaling. n PAX5 may serve as a chemosensitive marker of ESCC.n