Crithidia luciliae are haemoflaggelate single celled protists. They are used as a substrate in immunofluorescence for the detection of anti-dsDNA antibodies. They possess an organelle known as the kinetoplast which is a large mitochondrion with a network of interlocking circular dsDNA molecules. After incubation with serum containing anti-dsDNA antibodies and fluorescent-labelled anti-human antibodies, the kinetoplast will fluoresce. The lack of other nuclear antigens in this organelle means that using C. luciliae as a substrate allows for the specific detection of anti-dsDNA antibodies.