The biological role for CA125 is still under investigation though several studies have demonstrated a relationship between CA125 and the immune system. In 2003 Kui Wong et al. proposed a role in immunity based upon the characterization of the N-oligosaccharide structures identified on CA125 [7]. This hypothesis was supported by the study of Patankar et al. where natural killer (NK) cell function was found to be inhibited in the presence of CA125 purified from OVCAR-3 cell culture [8]. Cell membrane-bound CA125 was subsequently shown to bind directly to NK cells derived from peripheral blood of ovarian cancer patients and pregnant women [9]. A potential link between the suppression of NK cells in feto-maternal tolerance and the immune evasion of ovarian cancers was therefore proposed [9]. Further investigation into the mechanism of interaction revealed that binding was established through the sialic acid-binding Ig-like lectin-9 (Siglec-9), an inhibitory receptor expressed on NK cells [10,11].