The targeting vector contains an FRT site followed by PGK-puromycin, exon 11 of the mouse Npm1 gene, a polyA sequence, and a 4xSv40pA STOP cassette and a second FRT site inserted into the exon 10 intron, upstream of a humanized copy of exon 11 incorporating a TCTG frameshift duplication (p.W288fs*12; also called Npm1c Type A).