Mutant loxP sites and mutant loxP vectors. (a) The scheme of mutant loxP system is shown with sites for the rearrangement of mutant loxP sites. Nucleotide changes in mutant loxP sequences are stressed by stars. (b) Maps and positions of multiple cloning sites in the mutant loxP vectors, pLoxNeo, pLoxPuro and pLoxBsr. Each of the mutant loxP-flanked selectable marker genes can be utilized as a BamHI cassette. pLoxNeo, pLoxPuro and pLoxBsr have unique SpeI sites outside of mutant loxP-flanked region. pLoxPuro and pLoxBsr have unique NheI sites inside of mutant loxP-flanked region. These SpeI and NheI sites are important for the combination of the mutant loxP vectors with pExpress (Figure 3). Location of the PCR primers specific for the selectable marker genes (NE2, PU4, BS1) are shown by triangles.