In human cells, DNA double-strand breaks (DSBs) are repaired primarily by the DNA end-joining (EJ) process and thus, abnormal DNA EJ activities lead to an accumulation of mutations and/or aneuploidy, resulting in genetic instability of cells. Since genetic instability is the hallmark of cancer cells, we studied the DNA EJ activities of normal, non-malignant immortalized and malignant human epithelial cells to investigate the association between DNA EJ and carcinogenesis.