how to identify whether the PUC18 plasmid is good or not - about the PUC18 (Jun/01/2002 )

In theory, the color of PUC18 which live on Xgal plate should be blue. But  in my experince, it shows white and some show light blue. who can tell me why they can live on Xgal but can't show deep blue what books said.

Hi,
I think it depends on your X-gal and IPTG.
You should check the concentration and quality of them is OK and do it until all the blue colony show up at the room temperature.
Be careful to treat the X-gal, it is very sensitive to light.