Abstract: The invention pertains to a set of vectors (or of DNA fragments to be inserted in a vector) which distinguish from one another in that, taking into account one vector in which the number of pairs of bases between the reading initiation point of the vector and a point corresponding to the first pair of bases of a recognition site corresponding to a predetermined restriction enzyme, the two other vectors comprise between the corresponding points additional groups of pairs of bases comprising respectively two pairs of bases on the one hand and either one or four pairs of bases on the other hand, plus possibly whole numbers of triplets. On inserting a determined DNA fragment of which the expression is sought in bacteria in the three vectors, the reading of said DNA fragment will occur in phase as concerns one of the so modified vectors after transfection of bacteria therewith.