in the following example the direct selection for dhfr production was used 34 amplification 34 refers to the increase or replication of an isolated region within a cell 39 s chromosomal dna amplification is achieved using a selection agent e g methotrexate mtx which inactivates dhfr amplification or the making of successive copies of the dhfr gene results in greater amounts of dhfr being produced in the face of greater amounts of mtx amplification pressure is applied notwithstanding the presence of endogenous dhfr by adding ever greater mtx to the media amplification of a desired gene can be achieved by cotransfecting a mammalian host cell with a plasmid having a dna encoding a desired protein and the dhfr or amplification gene so that cointegration can occur one ensures that the cell requires more dhfr which requirement is met by replication of the selection gene by selecting only for cells that can grow in successive rounds of ever greater mtx concentration so long as the gene encoding a desired heterologous protein has cointegrated with the amplifiable gene replication of this gene gives rise to replication of the gene encoding the desired protein the result is that increased copies of the gene i e an amplified gene encoding the desired heterologous protein express more of the desired heterologous protein essentially this procedure was followed during the hhgf expression as shown in the following example suitable host cells for expressing the hhgf encoding dna include monkey kidney cvi line transformed by sv40 cos 7m atcc crl 1665 human embryonic kidney line 293 graham f l et al j gen virol 36 59 1977 baby hamster kidney cells bhk atcc ccl 10 1 chinese hamster ovary cells dhfr described by urlaub and chasin proc natl acad sci usa 77 4216.