Add reference LC-MS analysis of 18 OGE fractions optained from normally grown U2OS cell lysates. The generation of mathematical models of biological processes, the simulation of these processes under different conditions, and the comparison and integration of multiple data sets are explicit goals of systems biology that require the knowledge of the absolute quantity of the system’s components. To date, systematic estimates of cellular protein concentrations have been exceptionally scarce. Here, we provide a quantitative description of the proteome of a commonly used human cell line in two functional states, interphase and mitosis. We show that these human cultured cells express at least 10000 proteins and that the quantified proteins span a concentration range of seven orders of magnitude up to 20000000 copies per cell. We discuss how protein abundance is linked to function and evolution. Regarding the bioinformatics pipeline employed: the acquired raw files were converted to the centroid mxxml and mgf-format using MassMatrix mass spectrometric data file conversion tools (version 3.9, http://www.massmatrix.org) and searched using MASCOT (version 2.4) against a decoy database of the predicted proteome from homo sapiens extracted from the UniProt/SwissProt database (downloaded on 16/05/2012) and including known contaminants such as keratins/trypsin. The search criteria were set as follows: full tryptic specificity was required (cleavage after lysine or arginine residues); 3 missed cleavages were allowed; carbamidomethylation (C) was set as fixed modification; oxidation (M) as variable modification. The mass tolerance was set to 15 ppm for precursor ions and 0.5 Da for fragment ions.