In addition to QconCATS, pure proteins with well characterized modifications can be used to evaluate enrichment and detection of phospho-or glycoproteins, such as β-casein for phosphoproteomics or ribonuclease B (high mannose type glycans), and bovine fetuin (complex, sialylated glycans) for glycoproteomics, though their lack of complexity limits their utility in providing more complex QC metrics. To generate a larger number of peptides than a single protein, and provide a useful QC material for protein separation or fractionation methods, mixtures of a limited number of purified proteins have been designed as standards for proteomics and made commercially available. These include the above-mentioned UPS1 and UPS2 standards of 48 proteins, as well as the simpler UPS3 standard of eight non-human proteins spanning four orders of magnitude in concentration. To evaluate protein inference in downstream data analysis workflows, mixtures of proteins with known shared and unique peptides can be used. Examples of such mixtures include those used in the 2016 ABRF Proteome Informatics Research Group (iPRG) study . These protein/proteome mixtures are more complex, and their analysis less straight-forward than simple peptide mixtures. Unless isotopically labeled, they likely also interfere with endogenous proteins in experimental samples and are therefore not suitable spikes. Their direct support by data analysis software is also less trivial than the simple peptide mixtures, as they do not necessarily contain a small set of peptides expected to be observed in each run. More examples are listed in Table .