Oesophageal cancer is globally ranked as the ninth highest cancer, with a 5 year survival of less than 25%. Previously we reported altered glycosylation of serum complement component C9 in oesophageal adenocarcinoma (OAC). C9 is the terminal protein of the complement pathway, which polymerizes to form the membrane attack complex (MAC) leading to cell lysis. Intriguingly, we detected C9 protein in OAC tissues, including high level deposition in high grade dysplasia. These results led us to hypothesize that differentially glycosylated C9 promotes OAC proliferation and progression, instead of cell lysis. In this study, we investigated the C9 glycosylation sites associated with OAC, and developed recombinant glycosylated C9 mutants for functional analysis on OAC cells. Glycopeptide and glycoform analysis of immunopurified C9 from patient sera showed a 30% increase in the dual occupancy of W27 and W30 C-mannosylation in OAC patients compared to healthy controls. We expressed his-tagged glycomutant C9 in HEK 293 cells (W27F and W30F), and purified by His tag. The impact of differential C-mannosylation on C9 lytic and/or proliferative function was assayed by dose titration of C9 and cell viability analysis. Endocytosis and shedding of the MAC from the plasma membrane was also examined. In summary, this project determined the regulatory role of C9 C-mannosylation in OAC development.