Chemoproteomic workflow for childhood ALL surfaceome mapping. Relevant human samples were selected from the biobank of cryopreserved bone marrow aspirates collected at diagnosis (1) and expanded in immunodeficient mice (2). Subsequently, selective chemical tagging was used to enrich for cell surface–exposed glycoproteins (3). Mild oxidation of glycan residues on the surface of intact cells (A), followed by their biotinylation (B), membrane disruption, and protein digestion, allowed for affinity enrichment of selectively tagged glycopeptides (C). The peptide fraction stripped from glycan residues by enzymatic elution from streptavidin-coated beads was analyzed by mass spectrometry, resulting in protein identification (D). The established leukemia surfaceome data repository includes detailed information on identified cell surface–exposed proteins and respective peptides (4).