DNA-dependent protein kinase (DNA-PK) is made up of 3 subunits, 2 Ku proteins and a large catalytic subunit (DNA-PKcs). DNA-PKcs falls into the phosphatidyl-inositol 3-kinase superfamily althoμgh it does have serine/threonine protein kinase activity. This subunit migrates on SDS-PAGE gels as an intact and smaller processed form. In vitro, DNA-PK phosphorylates several transcription factors and other DNA-binding proteins and is believed to play a role in DNA damage recognition, repair and transcription. There is evidence to sμggest that DNA-PKcs may be a critical target for proteolysis by a caspase in apoptosis.