Radioprotection by CD31⁺ Lin^– c-kit⁺ Sca-1^– cells. (A) Left panel, adult bone marrow showing CD31⁺ lineage-negative cells (60%) and CD31^– lineage-negative cells (40%). Lineage-negative fraction is 3% of total BM. Right panel, cell-sorting gates for populations of CD31⁺ Lin^– cells based on c-kit and Sca-1 expression. (B) Lethally irradiated recipient mice (1200 cGy) received transplants of either of 2 populations of twice-sorted CD31⁺ cells: 10 000 CD31⁺ Lin^– c-kit^– cells (sort gate 2, solid line) and either 5000 (▵) or 10 000 (•) CD31⁺ Lin^– c-kit⁺ Sca-1^– cells (sort gate 1). Survival was monitored daily. Percentage of mice surviving is indicated. Pooled data from 3 independent experiments are shown (n = 9-17 mice per group).