There are many reasons to introduce new genes on recombinant vectors into mammalian cells. For example,in situstudies of the effects of particular gene products on DNA replication (1 –3 ), repair (4 –6 ), recombination (7 ), or gene expression (7 –11 ) can be performed. Alternatively, vectors that specifically respond to a particular enzymatic function, such as homologous recombination (12 ,13 ) or mismatch repair (14 ), can be introduced to determine whether a cell type of interest is positive or negative for that function.
