JAK2^V617F transgenic mice have markedly increased progenitor cells numbers and form autonomous hematopoietic colonies. Progenitor cell assays were carried out in MethoCult methylcellulose media in the presence or absence of Epo alone or a 4-growth factor/cytokine cocktail (mSCF, mIL-3, hIL-6, hEpo; A,B) or in MegaCult-C collagen-based media supplemented 10 ng/mL rm IL-3 and 20 ng/mL rh IL-6 with or without 50 ng/mL rh Tpo (C). Nucleated cells from peripheral blood (10⁵), spleen (10⁵), and bone marrow (2 × 10⁴ for A, 10⁵ for B and C) of control and transgenic line A mice were seeded, and colonies were scored after culture for 10 (A), 2 (B), and 7 (C) days. Data represent at least 3 independent experiments.