Schematic representation of the HERV-K113 provirus and structure of the oricoHERV-K113_GagProPol construct. To express high levels of the original HERV-K113 Gag, Gag-Pro and Gag-Pro-Pol proteins, a partially codon-optimized sequence (gray areas) encoding the reconstituted amino acid sequence of the virus was cloned downstream of the CMV promoter in the pcDNA3.1 vector. The 16 identified and reverted post-insertional amino acid changes are listed next to the oricoHERV-K113_GagProPol structure. Their positions in the open reading frames are indicated underneath. Numbers above refer to nucleotide positions of the codon-optimized regions starting with the first nucleotide of gag.