OC125-like and M11-like antibodies are proposed to detect a cysteine enclosed loop (C-loop) structure in the repeat domain [18,31,32] with a paucity of glyco-structures in the immediate proximity [18]. Extrapolated from the C-loop of a murine derived CA125 repeat domain homolog, Maeda et al. suggested that this antigenic region forms a β-sheet structure [22]. This was later confirmed with a synthesized 21 amino acid long human CA125 antigenic domain [23]. Here, a serine in position 8 of the loop was found to be essential for structural formation [23]. However, it is important to note that this serine at position 8 is conserved in the repeat sequences, throughout human CA125, in only around 25% of the cases. In approximately 60% of repeat sequences a proline is found at position 8, which results in random coil formation [23].