Hi Alice

You should check carefully what are the aminoacids involved. Of course that 2 additional aminoacids or 1 deletion could not be enough to explain many physiological changes, but in some cases they can be relevant. In the N-term of the protein two additional aminoacids could change the localization of the protein within the cell, but they can also be involved in interactions or post-translational modifications.

In the particular case of the estrogen receptor in humans, it is known as described in Uniprot, that the first 184 residues of the protein are involved in the interaction with MACROD1.... So, perhaps those 2 additional residues can modify the interaction properties of the protein.

Hope it helps. Best of luck

Paco