This exercise was designed to provide an exciting introduction to specific gene structure and function. In the experiment, students are given two plasmids (A and B) which are identified in the instructors guide. One plasmid (A) has a functional gene for the enzyme ß-galactosidase. The ß-galactosidase gene in the other plasmid (B) is inactive because it contains a segment of foreign DNA. In the first part of the exercise, students analyze restriction digests of both plasmids in order to determine which plasmid should have a functional ß-galactosidase gene. In the second part of the exercise, the plasmids are introduced into E. coli by transformation and the color of the resulting colonies (blue or white) is then used to assess the functional status of the ß-galactosidase gene. This exercise can be completed within a single 3-hour laboratory session or two 2-hour laboratory periods.