The fourth report from ISOBM TD-1 workshop described the use of a recombinant sequence of the 11th repeat from the N-terminus (R11) to test the binding capacity of various OC125-like, M11-like and OV197-like antibodies [33]. It was shown that the binding of the various monoclonal antibodies differed between the groups and subgroups. Within group A, subgroups A3 and A4 showed high binding to R11. Group B also displayed high reactivity with the exception of four investigated M11-like antibodies that showed low to no binding [33]. Additionally, group C antibodies had shown only little or no reactivity [33]. From those results it was concluded that the poor or absent binding of some antibodies towards R11 is unlikely to be due to a disparate sequence of this particular repeat to the other +60 repeat sequences, as there are only three main epitope families identified to date [33]. This observation was thought to be, more likely, a result of the single R11 repeat not exhibiting an optimal conformation as it is no longer in its native context [33]. This was exemplified by the binding behavior of OC125, which binds poorly to R11 [33] yet shows strong binding to a recombinant 3 tandem repeat form of CA125 [34].