A method for expression of transcribable unit(s) in a target cell is provided. The method comprises the steps of: a) providing a target cell expressing a site-specific recombinase, b) providing a DNA vector characterized by a 5′ to 3′ vector sequence orientation. The DNA vector comprises a plurality of recombination units, wherein a single recombination unit comprises at least one transcribable unit and a first type and a second type of target site that are recognizable by the site-specific recombinase. Recombination can only occur between two target sites of the same type and the first type of target site is located at the 5′ start of the recombination unit and the second type of target site is located at the 3′ end of the recombination unit. For all recombination units comprised within the DNA vector, the orientation of all of the first type of target sites are the same, and the orientation of all of the second type of target sites are the same. Step c) comprises introducing the DNA vector into the
