An ApR gene cassette was constructed using the gene present in the PBR322 derivative pTZ18. The cassette is maintained in the plasmid pcLINK-1 and may be excised with any of the six rare cutting enzymes NotI, SacII, MluI, SplI, BssHII or NarI. By using a double-digestion procedure, the ApR gene may be excised with two different protruding ends. In the process of constructing the pcLINK-1 plasmid the multiplecloning site of pTZ18 was extended with recognition sites for the enzymes NotI, SacII, MluI, SplI, BssHII and NarI.