Definitive proof of function in biological systems requires genetic analysis. 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 function. Changing the pattern of gene expression through random mutagenesis or by introducing expression libraries, followed by selection of mutant or variant cells and identification of a missing or overexpressed protein, has the power to reveal or confirm the roles of specific components of signaling pathways and to provide mutant cell lines and cDNA reagents to be used in defining detailed mechanisms through structure-function analyses. These examples of forward genetics contrast with reverse genetic approaches, where the function of a known gene product is explored by knockout or replacement. Here we review a broad range of techniques that have been used to alter gene expression randomly in mammalian cells, with examples of specific discoveries that have resulted from these applications of forward genetics.