And these molecules are known as Cofactors. So Cofactors are helping molecules that are needed for the enzymes to actually function effectively and efficiently. So when an enzyme is not bound to its Cofactor, we call the enzyme apoenzyme. But when the Cofactor is bound to the APO enzyme, we call that a hollow enzyme. So the hollow enzyme is simply an enzyme bound to its Cofactor. Now, we have many, many different types of Cofactors, as we'll eventually see.