Tryptophan must bind onto the inactive form of that repressive protein to activate it. And only then will that repressive protein bind onto the operator and inhibit the expression of these genes. And therefore inhibit the expression, the formation of tryptophan. So let's look at the two cases when our opera is on and when the opera is off. So let's begin with A. So let's suppose inside the cell we have very little tryptophan present and this is usually the case.