a member of a particular family (JAK) of tyrosine kinases involved in cytokine receptor signaling. Mutations in JAK3 on chromosome 19p cause autosomal recessive severe combined immunodeficiency [MIM*600173]. JAK1 and JAK2 are mapped to the short arms of chromosomes 1 and 9, respectively. [Janus, Roman god of doorways, depicted with two faces looking in opposite directions]
