A single loxP site was inserted upstream of exon 18 and a floxed hygromycin resistance cassette was inserted downstream of exon 19. The hygromycin resistance cassette was selectively excised from correctly targeted ES cells by transient transfection with a cre-expression vector. Exons 18 and 19, encoding part of the ATPase-helicase motifs, were thus flanked by loxP sites in the resulting ES cells.