Figure 30-7: G-region inversion. The alternative expression of S U and SU genes, due to the programmed inversion of the 3 kb DNA segment between the IR sites, allows the synthesis of an alternative set of tail fibers proteins.  The S gene of bacteriophage Mu is formed by a Sc sequence common for both S and S genes, localized before the IR.L site, and a variable portion Sv or Sv, localized within the invertible G-region and followed by the U and U genes, respectively.