You are studying a disease that occurs in individuals when an ER transmembrane protein called D2R is not inserted into the ER membrane of neurons. You find that the D2R gene is transcribed and the mRNA is exported out of the nucleus. You also notice that all other ER transmembrane proteins are inserted into the ER membrane properly. Based on what you have learned about RNA and protein processing in Lectures 9-13, what could be a cause of this disease