Ku antigen consists of two polypeptides （70 kDa and 80 kDa； the p80 subunit is occasionally called p86）. The Ku heterodimer provides DNA-binding and regulatory functions to DNA-dependent protein kinase which is a central enzyme for DNA recombination processes in eucaryotic cells. The Ku protein plays important roles in multiple nuclear processes including DNA repair， V（D）J recombination， telomere maintenance， and the regulation of specific gene transcription.
Antibodies to Ku were originally reported in scleroderma-polymyositis overlap syndrome （up to 20% of patients） but have also been detectable in about 20 % of patients with primary pulmonary hypertension （as a complication of overlap）， in 5-10 % of SLE and MCTD patients， and in up to 20 % of patients with primary Sj?gren’s syndrome.
In IIF using HEp-2 cells， Ku-positive sera yield a dense， fine granular pattern with nucleolar staining that is typical but not specific for anti-Ku. Detection of Ku antibodies by Western blot may bear false-negative results since the main reactive epitope on the p70/p80 heterodimer is conformation-dependent.
