To target the Apc locus, we obtained BAC clone RP23-233F17 that contains all the sequence of Apc except for the 5′ UTR and exon 1. Using a recombineering-based method for generating conditional mutations [12,13], subcloning of BAC and further modifications were conducted. The genomic sequence encompassing Apc exons 11 to 15 was first subcloned into pBR322 vector, and then a loxP site was introduced into intron 13 followed by loxP-FRT-PGKneor-FRT selection cassette into intron14 of the Apc gene (Figure 1A).