A team revealed that Ragnase-1 is a key regulator of the self-renewal and differentiation of hematopoietic stem and progenitor cells. Knockout of this gene in mice resulted in aberrant blood cell composition and hematopoiesis, and caused health-related outcomes such as low hemoglobin, enlarged spleen, and early death. This insight into blood cell homeostasis and differentiation could lead to treatments for diseases such as leukemia in which these regulatory processes go awry.