Affiliation: | ‡Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, the §Department of Pathology, Beth Israel-Deaconess Medical Center and Harvard Medical School, Boston, Massachusetts 02215, and the ¶Departamento de Gastroenterología, Facultad de Medicina, Pontificia Universidad Católica, Marcoleta 367, Santiago, 833-0024 Chile |