CD31 expression on hematopoietic stem cells in fetal liver and adult bone marrow. (A-B) Forward and side scatter (region 1, panel A) and lineage-negative (region 2, panel B) sorting gates. (C) Left panel, lineage-negative cells sorted based on c-kit and Sca-1 expression (KSL cells defined by region 3). Right panel, isotype control. (D-F) Lineage-negative cells sorted based on c-kit and Sca-1 expression from (D) day-14.5 fetal liver, (E) bone marrow from an 8-week-old mouse, and (F) bone marrow from a 24-month-old mouse. Right panels show CD31 expression on twice-sorted KSL cells. The percentage of KSL cells that are CD31⁺ is indicated. Representative data from 4 independent experiments are shown. Region 3 represents between 5% and 15% of lineage-negative c-kit⁺ cells. Region 4 encompasses a lineage-negative, c-kit^–, Sca-1⁺ cell population that increases with age.