Previous studies have established pivotal roles for c-Myc and its homolog N-Myc in hematopoietic stem cell (HSC) maintenance and niche-dependent differentiation. However, it remains largely unclear how c-Myc expression is regulated in this context. Here, we show that HSCs and more committed progenitors express similar levels of c-myc transcripts. Using knock-in mice expressing a functional enhanced green fluorescent protein-c-Myc fusion protein under control of the endogenous c-myc locus, c-Myc protein levels were assessed. Although HSCs express low levels of c-Myc protein, its expression increases steadily during progenitor differentiation. Thus, mRNA and protein expression patterns differ significantly in stem/progenitor cells, suggesting that c-Myc expression is largely controlled posttranscriptionally. Moreover, interferon-α exposure, which activates dormant HSCs, strongly induces c-Myc expression at the protein level but not at the transcript level. This posttranscriptional mechanism of c-Myc regulation provides the blood system with a rapid way to adjust c-Myc expression according to demand during hematopoietic stress. © 2014 by The American Society of Hematology.