Avoidance of graft-vs.-host disease (GVHD) after a hematopoietic stem cell transplant could depend on certain members of the microbiome. According to a study led by scientists at the Fred Hutchinson Cancer Center (FHCC), while some species of intestinal bacteria repressed the expression of the major histocompatibility complex II (MHC-II), others induced it and triggered the immune response that produces GVHD.