The identification of individual proteins in a biological sample, often coupled with determination of their relative abundance. It is a crucial step in analysing the proteins synthesized by cells or tissues (see proteomics). Various techniques are applied, most of which employ gel or liquid electrophoresis in combination with mass spectrometry to separate the proteins and determine their relative amounts. Samples can be labelled differentially with isotopic or fluorescent tags to pinpoint differences in protein profiles when comparing samples—e.g. from normal tissue and from a tumour—to assess abnormalities.