Indirect immunofluorescence staining pattern of anti-Rib-P-positive samples. One anti-Rib-P-positive serum that did not have autoantibodies to other known antigens (a-c) and the Centers for Disease Control and Prevention (CDC) anti-nuclear antibody reference serum number 12 (d-f) were tested at dilutions of 1:500 and 1:100, respectively, on slides from three different suppliers. Significant differences were observed in patterns of staining for the monospecific anti-Rib-P sera (a-c) on HEp-2 substrates from INOVA (San Diego, CA, USA) (a), ImmunoConcepts (Sacramento, CA, USA) (b), and Euroimmun (Lübeck, Germany) (c). Furthermore, the indirect immunofluorescence of the high-titer CDC anti-Rib-P reference serum produced only weak cytoplasmic staining on HEp-2 substrates from the same manufacturers (d-f). Rib-P, ribosomal P protein.