The function of specific eukaryotic DNA sequences can be assessed by the following procedure. First, the gene of interest is purified, usually by cloning in bacteria, and its structure is determined. Second, biochemical techniques are used to introduce mutations into the regions of interest. Finally, the normal and mutant genes are introduced into an appropriate eukaryotic cell where their expression can be examined in detail. In this paper I review some of the techniques that can be used for gene transfer, and speculate on how they might be applied to parasites.