Construct: Nucleotide substitution: G?A. Nucleotide substitution: A?C. Nucleotide substitution: G?T. Nucleotide substitution: C?G. Nucleotide substitution: A?G. Nucleotide substitution: A?T. Nucleotide substitution: T?C. Nucleotide substitution: T?A. Nucleotide substitution: T?C. The VC element is replaced with random nucleotides at 14 out of 18 positions, and the M1 3' splice site and polypyrimidine tract are replaced with the analogous sequences from the ftz region.