The inverted repeats of Tn5 are functionally different. One repeat codes for larger polypeptides, which are required for transposition. The other repeat has a better promoter for the neomycin resistance gene in the region of the repeat near the unique sequences. These dissimilarities are now shown to be caused by a single base pair difference. This change both creates a better promoter sequence and codes for part of a new UAA nonsense codon. Mutants in which the DNA sequence of a repeat is altered only at this base pair are shown to function like the opposite repeat. Furthermore, it is possible to suppress the UAA nonsense codon with an ochre suppressor, making the previously abbreviated polypeptides functional in transposition.