A loxP-flanked transcriptional stop signal in antisense orientation was inserted into intron 6, exon 7 was fused to human WT1 (lysine, threonine, and serine version termed +KTS) with a C-terminal FLAG tag, and a 3' neo cassette, replacing exons 8 through 11, was inserted by homologous recombination. The absence of endogenous protein product was confirmed by western blot analysis.