Sequence from an ends-out donor construct designed to target for has been inserted at the 5' end of the for gene (without the deletion of the for locus that was expected if successful homologous recombination had occurred). The inserted sequence consists of (in 5'-3' orientation upstream of for) 1: a 5kb fragment of sequence from upstream of for, 2: a loxP cassette that contains a w+mW.hs marker, 3: a single loxP site, 4: a 5.2kb nt of sequence from downstream of for and 5: an RS(I-SceI) site.