The invention provides methods, kits, and compositions comprising novel
mutant loxP sites. Such sites are particularly useful for procedures
requiring multiple site-specific recombination reactions, including
deletions or insertions of multiple genes or other sequences in the same
organism, staged insertions or deletions of genes of the same organism at
different times, assembly of large polynucleotide constructs by serial
site-specific recombination, and the like. In one aspect, compositions of
the invention includes particular mutant spacer oligonucleotides of loxP
recombination elements, the recombination elements themselves, and pairs
of non-promiscuous mutant loxP sites.