Two modifications of the GnRHR associated with altered expression. Addition of K191, an “extra” amino acid that appears in humans and other primates is associated with diminished receptor expression at the plasma, membrane. Pre-mammalian animals (fish, birds) have a C-terminal tail extension, absent in mammals, that enhances expression of the receptor. Accordingly, the tendency is to evolve a receptor that can be more closely regulated due to diminished expression.