What does '$3$' represent in the given figure?

Human gene expression is regulated when steroid molecules bind to:

In the $lac$ operon model,lactose molecules function as:

What is the location of the product of the $Lac \, y$ gene?

When do bacteria $NOT$ produce $\beta$-galactosidase?

The end product of a metabolic pathway binds to the operator to make it more active,so that it can easily bind with the repressor. In this case,the end product is called a/an ......