Because of the reciprocal difference in levels of proteoglycan 4 and plasma protease C1 inhibitor between both groups of breast cancer patients compared to the controls, calculated ratios of abundances of the two serum proteins further amplified their differences, with more than 5-fold difference for stage 0 and 4.17-fold difference in case of stage I (Fig 4, panel (c)). Here, PRG4 is linked to breast cancer.