A loxP site was inserted into intron 10 and a puromycin resistance gene cassette, a second loxP site and a modified exon 11 into intron 11. The exon modification involves replacing the end of the endogenous coding sequence and start of 3' UTR with the equivalent sequence for the human A variant of NPM1, which has a frameshift and slightly longer translated sequence: mouse sequence GCAGTGGAGGAAATCTCTTTAAGAAAAGG (reverse strand) was replaced with human sequence TCTGGCAGTGGAGGAAGTCTCTTTAAGAAAATA. (J:175778)