B6: Effects of DR and age on in vivo functional abilities of BMCs and numbers of LT-HSCs. (A) Total marrow functional ability based on the average composition of erythrocytes and lymphocytes. (B) Total marrow functional ability expressed in repopulating units. (C) Numbers of LT-HSCs present in the original donor marrow using the mKSL criterion (*P < .05; ***P < .001). (D) Functional ability per LT-HSC in animals using the mKSL criterion (**P < .01). Assays were conducted as described in the legend to Figure 1, except that donors and recipients were B6 Gpi1^b and competitors were B6 Gpi1^a. A total of 5-7 donors of each type was used over the course of 2 experiments. Old versus young functional ability in Figure 3A was significant at P < .001. Error bars represent SE.