Vectors containing a prokaryotic dihydrofolate reductase gene transform Drosophila cells to methotrexate‐resistance.

Transformed Drosophila Kc cell lines, resistant to methotrexate, an inhibitor of de novo purine and pyrimidine synthesis, have been obtained by calcium phosphate transfection of plasmids containing a sequence coding for a methotrexate‐resistant dihydrofolate reductase enzyme (DHFR). The introduced DNA is stably maintained in the cells as head‐to‐tail multimeric structures of the initial DNA sequence even after several months of culture in the presence of the selective agent. The introduced sequences are present at a high copy number in the transformed cells and express cytoplasmic RNAs transcribed from the DHFR gene.