How many proteins are there in a typical mammalian cell?

There is a major disparity between the number of polysomal mRNA species found in mammalian cells and the number of polypeptides detected by high-resolution two-dimensional polyacrylamide gel electrophoresis. Here we show that technical factors are not responsible for the relative paucity of proteins, and that the translation products of rare mRNAs would be easily detectable if all mRNAs were translated in proportion to their abundance. We conclude that a large majority of rare mRNAs are translated at no more than a tenth the average translation rate, if they are translated at all. There may be no more than 2000 physiologically significant primary gene products (polypeptides) in a typical mammalian cell.