They were originally named QUAD RNA after they were discovered as four repeat elements in Escherichia coli intergenic regions.

[1] The family was later renamed Sib (for short intergenic abundant sequences) when it was discovered that the number of repeats is variable in other species and in other E. coli strains.

Each of the ncRNA contains a short stretch homologous to boxC, a repeat element of unknown function present in 50 copies or more within the genome of E.

[4] Sib RNA regulates the expression of a toxic protein in a type I toxin-antitoxin system similar to that of hok/sok andldr-rdl genes.

[2] When sib was deleted in multi-copy plasmids, the cells could not be maintained due to the toxicity of the unrepressed ibs protein.