TCF4 and gastrointestinal polyp: fl-Tcf4 and dnTcf7 transcripts are the predominant Tcf/Lef1 family isoforms expressed in the intestine, which explains the opposite intestinal phenotypes in mice deleted of Tcf7l2 (lack of cycling stem cells) versus Tcf7 (intestinal polyposis) (8, 21, 22).