JAK1 and acute lymphoblastic leukemia: Also, gain-of-function JAK1 mutations have been reported in adult T-cell precursor acute lymphoblastic leukaemia (T-ALL),20 and JAK3 mutations in a few cases of leukaemia and lymphoma.21 More recently, TYK2 mutations were described in T-ALL, particularly the V678F STAT3-activating mutation.22 In addition, chromosomal re-arrangements resulting in fusion proteins of JAKs with other transcription factors were found in patients with haematological malignancies.