Graft-versus-host-disease (GVHD) upon HSCT results in severe intestinal inflammation, and enhanced inflammatory responses and increased CD8+ T cell responses importantly contribute to GVHD.42 Reconstitution with cord blood cells of young infants is associated with reduced cytotoxic responses and GVHD in HSCT.43 These observations after HSCT are consistent with our observations on physiological intestinal immune constitution during ontogeny, and indicate the presence of consistent time-window dependent programs of immune (re)constitution.23,44. The gene discussed is CD8A; the disease is graft versus host disease.