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Background & objectives: The nature of adaptable change of B-cell lymphoma-2 (BCL-2) and/or Bcl2-associated X protein (BAX) gene expression in the human peripheral blood mononuclear cells (PBMCs) irradiated by radioiodine in thyroid diseases therapy is not fully understood. In this study, the alternation of apoptotic gene expression was evaluated while the PBMCs collected from healthy volunteers were irradiated by the radioiodine-131 (131I). Methods: Fasting blood samples were obtained from healthy volunteers. PBMCs from group 0 to 6 were incubated and exposed to different doses of 131I in cell suspension for 6, 12, 24 and 48 h. The apoptosis rates and expression of BCL-2 and BAX genes of PBMCs were examined. Results: The apoptosis rate in the human PBMCs was gradually enhanced after six hour irradiation. The values of BCL-2 and BAX gene expression in groups 1-6 were higher than in group 0 within 6 h of irradiation, and then, these were decreased gradually from 6 to 12 h. BCL-2 gene expression increased in groups 1-3 after 12 h irradiation, but there was no difference in groups 4-6. The ratio of BCL-2/BAX gene expression among groups 4-6 gradually decreased during the period from 6 to 12 h, and it was significantly lower than in the group 0 at 12, 24 and 48 h. Interpretation & conclusions: The expression of BCL-2 and BAX genes was initially upregulated following irradiation. Later, the balance of BCL-2/BAX genes expression was adjusted, and then, PBMCs underwent apoptosis at higher doses of radiation. [ABSTRACT FROM AUTHOR]