Some of them originate from animals with naturally occurring genetic disorders, like nude mice (Foxn1 Nu/Nu, with spontaneous deletion in forkhead box N1 gene), which have low levels of T lymphocytes; SCID (severe combined immunodeficiency) mice, depleted of functional B and T lymphocytes; or NOD/SCID (non-obese diabetic/severe combined immunodeficiency). This evidence concerns the gene FOXN1 and severe combined immunodeficiency.