The invention relates to a viral expression construct and related viral vector and composition and to their use wherein said construct and vector comprise elements a) and b):a) a nucleotide sequence encoding an insulin operably linked to a first promoter,b) a nucleotide sequence encoding a glucokinase operably linked to a second promoter and said viral expression construct and related viral vector comprise at least one of elements c), d) and e):c) the first and the second promoters are positioned in reverse orientation within the expression construct,d) the first and the second promoters are positioned in reverse orientation within the expression construct and are located adjacent to each other ande) the first promoter is a CMV promoter, preferably a mini CMV promoter.
