Glycosylated proteins are often overexpressed in tumor cells and thus could serve as tumor markers, especially those with the interesting molecule sialic acid as their sugar moiety. Scientists now report on a bioorthogonal labeling test for sialylated glycoproteins based on a glycoproteomics approach. This assay not only assesses the level of sialylated glycans in the tumor cell membranes, but also identifies up- or downregulated proteins directly in the prostate cancer tissue.