For example, mutant Jak1 and Jak3 mice have severe combined immunodeficiency (SCID), and Jak1 mutants also have neurological defects and poor survival past birth; knock out mutations in Jak2 are embryonic lethal, and mutations in the Jak family member Tyk2 result in poor response to pathogens [29]. Here, JAK2 is linked to severe combined immunodeficiency.