Disclosed are methods for isolating transformed host cells which contain a vector incorporating a nucleic acid insert comprising the steps of: (a) providing a vector which comprises (i) a cytotoxic gene permitting positive selection, (e.g. a methyltransferase such as MspI) arranged such as to be insertionally inactivated by the incorporation of a nucleic acid insert (ii) a gene which provides a host cell transformed with the vector with a protective effect against a selection agent, but which does not degrade the selection agent (e.g. an antibiotic resistance gene such as TetA(c); (b) cloning a nucleic acid insert into the vector; (c) transforming host cells with the vector; (d) selecting transformed host cells in a single homogenous selection step, which step does not include growing the host cells on an agar medium.
