A cysteine residue in the third membrane-spanning segment of the human D2 dopamine receptor is exposed in the binding-site crevice Javitch, J.A.; Li, X.; Kaback, J.; Karlin, A. Proceedings of the National Academy of Sciences of the United States of America 91(22): 10355-10359 1994 0027-8424 10.1073/pnas.91.22.10355 https://eurekamag.com/research/009/748/009748872.php