The main findings of this study were as follows: (1) PRO-C9 assay wastechnically robust; (2) PRO-C9 was measurable in human serum, (3) PRO-C9 was significantly elevated in patients with bladder, breast, colorectal, gastric, head and neck, lung, melanoma, ovarian, pancreatic, prostate and renal cancer compared to healthy donors, and (4) PRO-C9 showed AUROC values between 0.58 and 0.89 when separating patients with solid tumors from healthy donors, indicating its potential as a diagnostic biomarker. Here, C9 is linked to renal carcinoma.