A hitherto unrecognised subgroup of patients can now be identified, since the discovery of another, independent, serum marker for connective tissue diseases. Eighteen serum samples among more than 1700 tested routinely for antinuclear antibodies (ANAs), denoting connective tissue diseases, showed specificity solely for 52 kDa protein Ro52 and no cross reactivity with anti-SSA/Ro or SSB/La. None of them reacted with classic anti-SSA/Ro immunological methods—double immunodiffusion with thymus/spleen nuclear extract or natural Ro60, immunoblotting with natural or recombinant Ro60, and ELISA with natural SSA/Ro60 or recombinant Ro52 and Ro60 in unspecified proportions. Immunoblotting showed natural Ro60 as having just one 60 kDa protein band, which reacted exclusively with Ro60 monoclonal antibodies. However, 16 of the 18 sera reacted positively in ELISAs with equal amounts of recombinant Ro60 and Ro52, or Ro52 alone, or immunoblots with HeLa S100 substrate. Classic connective tissue disease was diagnosed for 12 of the 18 patients. The incidence of Ro 52 specific antibody was calculated at about 1% of serum samples positive for ANA.

In all, 1727 consecutive ANA positive serum samples were tested in parallel in double immunodiffusion against thymus/spleen nuclear extract and line immunoassay with recombinant Ro52, SSB/La, and natural Ro60. Samples that were positive for Ro52 alone were tested by a range of different methods to establish their specificity.

Anti-SS/Ro antibodies are the most prevalent ANAs, and they recognise Ro60 kDa protein; SSB/La antigen is also associated. Most anti-SS/Ro sera cross react with Ro52 kDa protein, but it was not known whether this protein was an independent marker.

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