The 23S rRNA is a 2904 nt long (in E. coli) component of the large subunit (50S) of the bacterial/archean ribosome. The ribosomal peptidyl transferase activity resides in domain V of this rRNA, and this domain is the most common binding site for antibiotics that inhibit translation. A well-known member of this antibiotic class, chloramphenicol, acts by inhibiting peptide bond formation, with recent 3D-structural studies showing two different binding sites depending on the species of ribosome. Linezolid and quinupristin-dalfopristin also bind to the 23S rRNA, and cross-resistance has been demonstrated between these antibiotics. Compared to 16S rRNA genes, 23S rRNA genes typically have higher sequence variations including insertions and/or deletions.