said:

You've already got it! beta-Galactosidase catalyzes the hydrolysis of X-Gal producing a blue precipitate that can be easily visualized under a microscope.
It acts as a reporter to determine the percentage of transfected cells (or a transgenic mouse with appropriate promoters) expressing the lacZ gene. The assay can be completed in 30 minutes, and the blue precipitate can take from 30 minutes to overnight to appear.