In this manuscript the authors takes a novel and clever approach to examining the proliferation history of hematopoietic stem cells. The strategy to label cells with NHS-biotin is appealing as it allows for functional evaluation of cellular properties based on differential cycling kinetics, which has not been possible in previous studies in which hematopoietic stem cell proliferation has been evaluated by for example BrdU incorporation.
It is interesting that the authors with this novel approach could establish that functionally defined hematopoietic stem cells do not remain quiescent for very long periods of time.