Allolactose

... consider this I know that in the presence of glucose ALONE we still get negligible lac products (let's say 0.5 units) because there is always some allolactose present (this is the inducer, it binds to the repressor and gets off the operator, thus allowing transcription) NOW, in an I(S) mutant, ...

See entire post

A small correction, though I guess a bit late for the question: Technically, allolactose is the inducer. e. coli converts a small amount of lactose to allolactose and this then binds to the repressor.

See entire post