An increased number of circulating CD34 þ hematopoietic progenitors with a prominent proliferation of the megakaryocytic (MK) population are the hallmarks of the myeloproliferation in myelofibrosis with myeloid metaplasia (MMM). Analyzing the potential contribution of the stem cell leukemia (SCL) gene in MMM myeloproliferation was doubly interesting for SCL is expressed both in primitive-uncommitted progenitor cells and erythroid/MK cells, its transcription differentially initiating from promoter 1b and 1a, respectively. Our results show that: (i) the expression of SCL transcript is increased in peripheral blood mononuclear cells (PBMCs) from patients; (ii) SCL gene transcription is altered in MMM CD34 þ progenitor cells sorted into CD34 þ CD41 þ and CD34 þ CD41 À subpopulations. Actually, in patients, SCL transcription initiated at promoter 1b is restricted to primitive CD34 þ CD41 À progenitor cells, while it is detectable in both cell subsets from healthy subjects; (iii) the full-length isoform of SCL protein is present in patients' CD34 þ cells and in PBMC; in the latter the SCL-expressing cells mainly belong to the MK lineage in which its sublocalization is both nuclear and cytoplasmic, which contrasts with the sole nuclear staining observed in normal MK cells. Our demonstration of altered expression and transcription of SCL in patients' hematopoietic cells emphasizes the possible contribution of this regulatory nuclear factor to the hematopoietic dysregulation, which is a feature of myelofibrosis with myeloid metaplasia.