Jingyi Jessica Li, Department of Statistics and Department of Human Genetics, University of California, Los Angeles, CA 90095, USA.
\nMark D. Biggin, Genomics Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA<\/p>\n
Abstract<\/strong><\/p>\n Mammalian proteins are expressed at \u223c103<\/sup> to 108<\/sup> molecules per cell (1<\/em>). Differences between cell types, between normal and disease states, and between individuals are largely defined by changes in the abundance of proteins, which are in turn determined by rates of transcription, messenger RNA (mRNA) degradation, translation, and protein degradation. If the rates for one of these steps differ much more than the rates of the other three, that step would be dominant in defining the variation in protein expression. Over the past decade, system-wide studies have claimed that in animals, differences in translation rates predominate (2<\/em>\u20135<\/em>). On page 1112 of this issue, Jovanovic et al.<\/em> (6<\/em>), as well as recent studies by Battle et al.<\/em> (7<\/em>) and Li et al.<\/em> (1<\/em>), challenge this conclusion, suggesting that transcriptional control makes the larger contribution.<\/p>\n