Vectors frequently expresses beta-galactosidase (lacZ) gene at the cloning sites. Insertion may disrupt the lacZ gene, thus generating white clonies on a X-gal plate.

1. Prepare X-gal plates.

2. Spread cells on the X-gal plates.

3. Incubate at 37 °C for overnight.

4. Blue clonies should appear overnight.

5. If the color is light, store plates in 4 °C, blue clonies get darker over time in 4 °C.