Immunofluorescence histochemistry depicting the proliferating cell nuclear antigen staining pattern. The antibody in this systemic lupus erythematosus serum reacts with proliferating cell nuclear antigen (PCNA), identified as an auxiliary protein of DNA polymerase delta that is involved in DNA synthesis. PCNA is seen to localize in nuclei that are in the S phase of the cell cycle, and the variation in size and density of speckled staining is related to early or late stages of the S phase. The nuclei of cells in interphase which are not in DNA synthesis do not react with the antibody. The cytoplasm was counterstained (orange) to delineate cytoplasm from the yellow–green speckled staining of PCNA particles in the nucleus.