To introduce the mutation of Apc into cells expressing K14, we crossed WW6 ES cell–derived [15] ApcCKO/+ mice with K14-cre recombinase mice in FVB background [16]. The K14-cre; ApcCKO/+ mice were normal in appearance and were fertile. K14-cre; ApcCKO/+ males were crossed to ApcCKO/CKO females to avoid the potential deleter effect in oocytes of K14-cre–positive females [17]. The mice were intercrossed thereafter for maintenance; hence, the mice used for analysis were in a mixed background of FVB, 129/S, and C57BL/6 in similar proportions, with minimal contribution of SJ.