Intestinal epithelial cell lineage differentiation (Fig. 1A) and proliferation (Fig. 1B,C) were found to be normal in Gpa33−/− mice and, apart from an extremely mild, progressive colitis with age, which is indistinguishable from that seen in WT, Gpa33−/− mice did not exhibit steady-state colitis (Fig. 1D,E). This evidence concerns the gene GPA33 and colitis.