The DNA-dependent protein kinase (DNA–PK) is a nuclear serine/threonine protein kinase that is activated upon association with DNA. Biochemical and genetic data have revealed DNA–PK to be composed of a large catalytic subunit, termed DNA–PKcs, and a regulatory factor termed Ku. In recent years, mammalian DNA–PK has been shown to be a crucial component of both the DNA double-strand break (DSB) repair machinery and the V (D) J recombination apparatus. In addition, recent work has implicated DNA–PK components in a variety of other processes, including the modulation of chromatin structure and telomere maintenance. Our DNA is constantly under attack from reactive oxygen intermediates—by-products of the oxidative metabolism we have evolved for energy supply. Reactive oxygen species are capable of producing DNA singlestrand breaks and, where two of these are generated in close proximity, DNA double-strand breaks (DSBs). In addition, single-and double-strand breaks can be induced when a DNA replication fork encounters a damaged template, and are generated by exogenous agents such as ionizing radiation (IR) and certain anti-cancer drugs (eg, bleomycin). DSBs also occur as intermediates in site-specific V (D) J recombination, a process that is critical for the generation of a functional vertebrate immune system. If DNA DSBs are left unrepaired or are repaired inaccurately, mutations and/or chromosomal aberrations are induced, which in turn may lead to cell death or, in extreme cases, cancer. To combat the serious threats posed by DNA DSBs, eukaryotic cells have evolved several mechanisms to mediate their repair. In higher eukaryotes, the predominant of these mechanisms is DNA nonhomologous end-joining (NHEJ), also known as illegitimate recombination. DNA–PK plays a key role in this pathway.

Early studies on DNA–PK focused primarily on its biochemistry, and led researchers to speculate on its function as a modulator of transcription. However, this viewpoint took a dramatic change when it was shown that DNA–PK is activated most potently by DNA DSBs, suggesting that it might play a role in recognizing DNA damage. This observation stimulated investigations into the potential role of DNA–PK in DNA repair and led to