They can be grossly divided into two groups: (1) models displaying a general immunodeficiency, such as Severe Combined Immunodeficiency Disease (SCID) or Recombination activating gene RAG1-/- mice that lack functional T-cells or B-cells, or (2) more refined models that target a specific gene implicated in the host response. Here, RAG1 is linked to severe combined immunodeficiency.