Age-related decline in the homing efficiency of murine HSCs. Lethally irradiated young or old Ly-5.1 mice were intravenously injected with 2-3 × 10⁶ old or young Ly-5.2 donor cells, as depicted in Figure 2. The number of CRUs recovered from the BM 24 hours later was determined by limiting-dilution competitive repopulation assays conducted in young secondary recipients assessed for donor engraftment at 5, 10, 17, and 26 weeks after transplantation (Tables 1, 2). The homing efficiency of HSCs was calculated by dividing the number of CRUs recovered in the primary BM 24 hours after transplantation by the number of CRUs initially injected and then multiplying by 100%. Three different transplantation groups—young cells into young recipients (□), young cells into old recipients (▦), and old cells into young recipients (▪)—are depicted. Data for young-to-old and old-to-young groups are derived from the experiments described in Tables 1 and 2. Data for the young-to-young group, though collected contemporaneously with those for the other transplantation groups, were reported previously by Szilvassy et al²³  and are shown for comparison. All values are mean ± SEM, and statistically significant differences are designated with an asterisk (P < .05).