How can genes be "inactivated by modifications in the nitrogenous bases"

That's written in my book, but it's not explained. :/

Any help appreciated.

Chemical modification of bases can either occur spontaneously or be induced by chemical mutagens. About 1000 chemical substances induce mutations in DNA.

http://www.scienceclarified.com/Ga-He/Ge…
Check this out, it's the basis of genetic engineering