Immunosuppression is required to produce a progressive high level of infection with M. abscessus in mice, either genetically, as evident in nude, severe combined immunodeficiency (SCID), interferon-gamma (GKO) and granulocyte-macrophage colony-stimulating factor (GM-CSF) knock-out mice, or chemically by dexamethasone treatment17,18. The gene discussed is CSF2; the disease is severe combined immunodeficiency.