How does lacZ staining work?

LacZ is a frequently used reporter gene, encoding for the protein beta-galactosidase in cultured cells, which appear blue when the cultured cells are grown on a medium containing X-gal analog.

• The staining solution is made of X-gal (200 mg/ml), MgCl2 (1M), K ferri-cyanide (50mM), K ferro-cyanide (50mM), and PBS.
• The cells are then fixated in formaldehyde/methanal (37 %), glutaraldehyde (25 %), and PBS.

The steps for lacZ staining are:

1. Wash cells in PBS
2. Add fixative solution
3. Incubate for 2 minutes
4. Wash 3 times with PBS
5. Add staining solution
6. Incubate overnight at 37°C