RAC1 and intestinal cancer: To test the functional significance of Rac1b expression in intestinal tumorigenesis we generated mice carrying a novel floxed allele permitting deletion of Rac1 exon 4 (Rac1bfl) (Figs. 3A and S3A) and bred these mice onto the tamoxifen-inducible VilCreERT2Apcfl/+ intestinal cancer model31.