TEL-Jak2 fusion proteins are constitutively tyrosine-phosphorylated in Ba/F3 cells. Ba/F3 (lanes 1, 2, 15, and 16), Ba/F3 HA-TEL-Jak2 JH1 subclone 18 (lanes 3, 4, 17, and 18), Ba/F3 HA-TEL-Jak2 JH1 subclone 36 (lanes 5 and 6), Ba/F3 HA-TEL-Jak2 JH2 subclone 4 (lanes 7 and 8), Ba/F3 HA-TEL-Jak2 JH2+JH1 subclone 23 (lanes 9 and 10), Ba/F3 HA-TEL-Jak2 JH2+JH1 subclone 31 (lanes 11 and 12), and Ba/F3 HA-TEL-KI-Jak2 subclone 11 (lanes 13 and 14) cells were depleted of cytokine and stimulated in the presence or absence of IL-3. Lysates were immunoprecipitated with an antibody that recognizes the Jak2 JH1 domain (pan Jak, lanes 1 through 6 and 9 through 14) or the Jak2 JH2 domain (Jak2, lanes 7 and 8) and tyrosine-phosphorylated proteins were detected by immunoblotting with a monoclonal antiphosphotyrosine antibody. Lysate controls are shown in lanes 15 through 18. The mobility of tyrosine-phosphorylated Jak2, HA-TEL-Jak2 JH1, and HA-TEL-Jak2 JH2+JH1 are indicated.