Proteomics is the study of the structure, function and interaction of proteins. Proteins play a critical role in the life of any organism by being involved in many processes within the cell. The proteome, which is the complete collection of proteins in an organism, is much more complicated to study than the genome or transcriptome. An organism harboring 20 000 genes can have more than 1 million different proteins due to alternative splicing and post-translational modifications. Furthermore, the level of transcription of a gene often does not reflect the level of the encoded protein. An mRNA can be present in numerous copies in a cell but the mRNA stability (half life) and efficiency of translation will affect the level of the corresponding protein. A protein can also be present in a cell but inactive until subjected to post-translational modifications like phosphorylation. Finally, many proteins can interact with other proteins and only the resulting complex may have biological function. The measurement of mRNA levels via transcriptomics (e.g., via the use DNA microarrays, see discussion of this subject on our website) is complimentary to proteomics.