There is described a nucleic acid molecule comprising a nucleotide sequence encoding for a functional preproinsulin protein wherein the nucleotide sequence has at least 86% identity to the sequence of SEQ ID NO. 1. Also described are: vectors comprising the nucleic acid molecule for expressing the preproinsulin protein； host cells comprising the nucleic acid molecule or a vector； a transgenic animal comprising cells comprising the nucleic acid molecule or the vector； a pharmaceutical composition comprising the nucleic acid molecule or the vector； a method of treating diabetes comprising administering a therapeutically effective amount of the vector to a patient suffering from diabetes； the nucleic acid molecule for use in therapy； and the nucleic acid molecule or the vector for use in the treatment of diabetes.
