The episomal inducible 4th generation of LEXSY makes use of amplification and oligomerisation of expression plasmids maintained extrachromosomally as self-replicating episomes in the LEXSY host cells, providing enhanced number of target gene copies for transcription by T7 RNA polymerase.
It was shown for a number of proteins, that the episomal inducible expression achieved 5 - 10-fold higher yields than integrative inducible expression. High expression strains are obtained by clonal selection and screening which is based on fluorescence intensity resulting from the co-expressed BleCherry, egfp or Ds-red genes. Primary screening on solid media can be performed in daylight since the colours are visible by eye.