Altered frequencies of HSCs and MPPs in aged BM. (A) BM was harvested from 2-month-old, 12-month-old, or 18- to 24-month-old mice and LSK cells were analyzed for Flt3 expression by flow cytometry. (B) Numbers of HSCs (defined as LSKFlt3⁻) and MPPs (defined as LSKFlt3⁺) in young and aged BM, calculated from flow cytometric plots in panel A. Numbers are expressed as total cells (counted from 2 femora plus 2 tibiae plus 2 humeri from each mouse) per gram of body weight. Data represent at least 6 mice per group. Error bars represent 1 SEM. (C) cDNA from total LSK cells was amplified in the presence of gene-specific FAM-labeled Taqman probes. Data are shown as expression relative to Hprt amplification (*P < .001). Error bars represent 1 SEM.