1. A nucleic acid containing at least one homing endonuclease (HE) site and at least one restriction enzyme (X) site wherein the HE and X sites are selected such that a ligation product having compatible cohesive ends results when the nucleic acid is cut by a homing endonuclease and a restriction enzyme, respectively, and the ligation product having compatible cohesive ends can neither be cleaved by the homing endonuclease nor the restriction enzyme, the nucleic acid comprising a nucleotide sequence selected from the group consisting of SEQ ID NO: 1, SEQ ID NO: 50, SEQ ID NO: 51, SEQ ID NO: 52 and SEQ ID NO: 53.