['Many studies have demonstrated that microRNA-21 (miR-21) acts as an oncogene in the tumourigenesis of a variety of tumours and may be involved in the chemotherapeutic drug resistance of tumour cells. In this study, we utilized the leukaemia cell line K562 as an in vitro cell model to investigate whether miR-21 is involved in X-ray irradiation resistance.', 'Retroviral transduction and antisense oligonucleotide transfection were used to overexpress or knock down miR-21 expression, respectively. An MTT assay was used to measure cell viability, and western blotting was performed to detect the expression of the miR-21 target gene, PTEN (phosphatase and tensin homologue), and its downstream signalling components, phosphatidylinositol 3-kinase (PI3K), and AKT.', 'The overexpression of miR-21 decreased the protein expression levels of PTEN, increased the phosphorylation level of AKT, and enhanced the X-ray irradiation resistance in K562 cells. In contrast, the knockdown of miR-21 increased the PTEN protein expression, reduced the phosphorylation levels of the AKT, and increased the sensitivity of K562 cells to X-ray irradiation. The overexpression of PTEN or the knockdown of AKT also increased the sensitivity of K562 cells to X-ray irradiation.'] Is microRNA-21 involved in X-ray irradiation resistance in K562 leukaemia cells?