Publisher Summary Long palindromic DNA sequences lead to instability, and when exceeding a certain threshold of about 150 bp, to nonviability of the carrier replicon in bacteria. The viability is restored by placing any non-palindromic DNA sequence at least 50 bp in length between the inverted repeats. This chapter discusses the principle to construct positive selection vectors and improve them by screening recombinants directly and using them for other applications. The plasmid p JOE930 was constructed in a single cloning step from pICI9H, and in the same way positive selection vectors can be obtained from any plasmid of the pUC series. The advantage of having a strong promoter on both sides of the integration site is to make the expression of an inserted gene orientation independent. This should halve the number of clones needed in cloning experiments, in which the gene must be identified by expression via an E. coli promotor. The vector p JOE773 with the transcription terminators flanking the insertion site is useful for the verification of the functionality of a promoter in an inserted DNA fragment.