ABSTRACT
Historically, the first observation that transfected heterologous DNA may translate into protein in a controlled way in a mammalian cell environment was by transient expression (1,2). Following this achievement, the search for novel genes and their associated gene product has been the major driving force for the development of several powerful transient expression systems. The complete sequencing of the human genome has then started the expected race to the next, more interesting level: the complete human proteome, comprising tens of thousands of gene products. Clearly, today, only a minority of these gene products is fully understood in terms of their biochemical and biological function. Transient gene expression bridges these two worlds in the most efficient manner. Nowadays, several transient expression systems have matured and are as diverse as direct DNA transfection into mammalian cells, recombinant virus infection, injection of Xenopus oocytes with mRNA or complete in-vitro coupled transcription-translation. Whereas functional expression of large cDNA libraries has allowed the cloning of many new genes by their function, the analysis of the gene products themselves, i.e., the corresponding proteins, however has lagged behind because of the lack of efficient, quick and easy ways of producing sufficient amounts.
