Determination of marker gene copy number by Southern blot hybridization. Southern-blot analysis of digested (PstI) genomic DNA from 9 randomly isolated paromomycin resistant mutants. A fragment of the AphVIII gene labeled with 32P-dNTPs was used as probe. As shown, most transformants have a single copy of the integrated marker gene. Only the transformant represented in lane 4 may have two insertions (indicated by the two hybridizing bands). p.s., parental strain used to obtain the transformants.