OBJECTIVE: This study aims to investigate effects of checkpoint kinase. mediator of DNA damage checkpoint 1 (MDC1) and p53-binding protein 1 (53BP1) silencing on p53, checkpoint kinase 1 and 2 (CHK1 and CHK2), and CHK2-T68 expression. MATERIALS AND METHODS: Eca109 cells were divided into untransfected Eca109, Blank-vector, MDC1-RNAi transfection, and 53BP1-RNAi transfection group. Streptavidin-peroxidase (SP) immunohistochemical assay was used to examine CHK2-T68 expression. About 4 groups were used to establish esophageal carcinoma nude-mouse models, and assigned as Eca-109 control (or Eca-109 plus 15 Gy y-rays irradiation, Eca-109+IR), Blank-vector (or Blank-vecor+IR), 53BP1-RNAi (or 53BP1-RNAi+IR), and MDC1-RNAi group (or MDC1-RNAi+IR group) by injecting. The expression of p53, CHK1. CHK2 were evaluated using SP immunohistochemical assay. RESULTS: 53BP1 and MDC1 down-regulation significantly inhibited expression of CHK2-T68 in Eca-109 cells compared to untreated group (p<0.05). There were significant differences for CHK2-T68 expressions in different time and groups (p<0.05). 53BP1 down-regulation significantly reduced p53 and enhanced CHK1 and CHK2 expression compared to that of Eca-109 control group (p<0.05) in Eca-109 cells. 53BP1 down-regulation significantly regulated CHK1, CHK2. and p53 in xenograft nude mice models exposed to gamma-ray irradiation compared to that of untreated group (p<0.05). p53 was negatively correlated with CHK1 and CHK2 in xenograft nude mice models. CONCLUSIONS: 53BP1 regulated the cell cycle arrest by modulating p53, CHK1, and CHK2 expression in both Eca-109 cells and xenograft nude mice models.