Purified proteins are used in various types of scientific experiments and fields. For example, in structural biology insights into the functional mechanisms can be obtained by elucidating the 3D molecular structure of proteins and protein complexes using technologies such as X-ray crystallography, cryo-EM and NMR. In biochemistry and biophysics, interactions with other proteins, nucleic acids and small molecules can be studied by determining affinities and specificities. Proteins can function as antigens to generate specific antibodies or as reagents in cell biology experiments. Furthermore, recombinant proteins can be used as tool molecules in genomics, chemical biology and microscopy assays. In order for these experiments to produce reliable and biologically relevant results, they must be performed using high-quality proteins that are active, properly folded, in the right oligomeric state and contain correctly inserted co-factors. Ensuring that these parameters are fulfilled requires quality control.