generation of mice , bone marrow transplantation , and mathematical modeling are described in supplementary  full methods  online .
hematopoietic stem cells ( hscs ) are thought to divide infrequently based on their resistance to cytotoxic injury targeted at rapidly cycling cells1 , 2 and have been presumed to retain labels such as the nucleotide analogue 5-bromodeoxyuridine ( brdu ) . 
 however , recently it has been demonstrated that brdu - retention is neither sensitive nor specific for hscs3 . 
 here we show that transient , transgenic expression of a histone2b ( h2b)-green fluorescent protein ( gfp ) fusion protein in mice allows superior labeling of hscs and permits improved analysis of their turnover in combination with other markers . 
 mathematical modeling of h2b - gfp dilution in hscs , identified with a highly stringent marker combination ( lk+s+cd48cd150+)4 , revealed unexpected heterogeneity in their proliferation rates and suggests that ~ 20% of hscs turn over at an extremely low rate (  0.81.8% per day ) . 
 prospective isolation and transplantation of lk+s+cd48cd150 + hscs with different h2b - gfp levels revealed that higher h2b - gfp label retention correlates with superior long - term repopulation potential .