Forward genetics in mammalian cells: functional approaches to gene discovery

Definitive proof of function in biological systems requires genetic analysi s. Only when the loss of a particular protein corresponds to the loss of a specific function can one be sure that the protein truly affects the functi on. Changing the pattern of gene expression through random mutagenesis or b y introducing expression libraries, followed by selection of mutant or vari ant cells and identification of a missing or overexpressed protein, has the power to reveal or confirm the roles of specific components of signaling p athways and to provide mutant cell lines and cDNA reagents to be used in de fining detailed mechanisms through structure-function analyses. These examp les of forward genetics contrast with reverse genetic approaches, where the function of a known gene product is explored by knockout or replacement. H ere we review a broad range of techniques that have been used to alter gene expression randomly in mammalian cells, with examples of specific discover ies that have resulted from these applications of forward genetics.