Methods, reagents and kits for diagnosing and treating cancer such as kidney or colorectal cancer are disclosed. An immunoassay for detecting kidney or colorectal cancer is based on the relative change of the ADAMTSL4 protein in urine or blood compared with normal tissue. An immunohistochemical assay for detecting kidney or colorectal cancer is based on the relative absence of labeled antibody binding to cancerous tissue, compared with normal tissue.