5-Aza-2'-deoxycytidine (5-aza-dC) a DNA methyltransferase inhibitor, exerts antitumor activity through induction of cell cycle arrest, apoptosis and DNA damage. In this study, we showed that MHC class I-related chain B (MICB), a ligand of the NKG2D receptor expressed by natural killer cells and activated CD8(+) T cells, was upregulated following 5-aza-dC treatment. The upregulation of MICB was accompanied by promoter DNA demethylation and DNA damage. Furthermore, the upregulation of MICB was partially prevented by pharmacological or genetic inhibition of ataxia telangiectasia mutated (ATM) kinase. Our results suggest that promoter DNA demethylation, in combination with DNA damage, contribute to the upregulation of MICB induced by 5-aza-dC. (c) 2008 Elsevier Inc. All rights reserved.