1. Cellular and Molecular Biology Graduate Program, University of Michigan, 2966 Taubman Medical Library, Ann Arbor, Michigan 48109-0619, USA
2. Stanford Institute for Stem Cell Biology and Regenerative Medicine and Division of Hematology/Oncology, Internal Medicine, Stanford University, 1050 Arastradero Road, Palo Alto, California 93404, USA
3. Department of Hematology/Oncology, Internal Medicine, University of Michigan, 1500 East Medical Center Drive, Ann Arbor, Michigan 48109, USA
4. Division of Hematology/Oncology, University of Rochester, Rochester, New York 14642, USA

Correspondence to: Michael F. Clarke² Correspondence and requests for materials should be addressed to M.F.C. (Email: mfclarke@stanford.edu).

Haematopoiesis is maintained by a hierarchical system where haematopoietic stem cells (HSCs) give rise to multipotent progenitors, which in turn differentiate into all types of mature blood cells¹. HSCs maintain themselves for the lifetime of the organism because of their ability to self-renew. However, multipotent progenitors lack the ability to self-renew, therefore their mitotic capacity and expansion potential are limited and they are destined to eventually stop proliferating after a finite number of cell divisions^{1, 2}. The molecular mechanisms that limit the proliferation capacity of multipotent progenitors and other more mature progenitors are not fully understood^{2, 3}. Here we show that bone marrow cells from mice deficient in three genes genetically downstream of Bmi1—p16^Ink4a, p19^Arf and Trp53 (triple mutant mice; p16^Ink4a and p19^Arf are alternative reading frames of the same gene (also called Cdkn2a) that encode different proteins)—have an approximately 10-fold increase in cells able to reconstitute the blood long term. This increase is associated with the acquisition of long-term reconstitution capacity by cells of the phenotype c-kit⁺Sca-1⁺Flt3⁺CD150^-CD48^-Lin^-, which defines multipotent progenitors in wild-type mice^{4, 5, 6}. The pattern of triple mutant multipotent progenitor response to growth factors resembles that of wild-type multipotent progenitors but not wild-type HSCs. These results demonstrate that p16^Ink4a/p19^Arf and Trp53 have a central role in limiting the expansion potential of multipotent progenitors. These pathways are commonly repressed in cancer, suggesting a mechanism by which early progenitor cells could gain the ability to self-renew and become malignant with further oncogenic mutations.