The par stability determinant is a 400 bp locus of the pAD1 plasmid which encodes a type I toxin-antitoxin system in Enterococcus faecalis.

[3] The par locus contains two genes: fst which encodes a 33-amino acid toxic protein and a gene for RNAII, the small RNA anti-toxin which inhibits fst translation.

[4] The two genes are found on opposite DNA strands and share a 5' region which is where they are thought to have an antisense interaction.

[4] Their RNA secondary structures have been predicted computationally, the complementary regions appear to be presented on exposed loops for interaction.

If a daughter cell does not inherit the par locus, the unstable RNAII will quickly degrade leaving the long-lived fst toxin to damage or kill the daughter cell.