SP analysis of subsets defined by CD34 expression intensity. Analysis of a representative (8-12 weeks) murine marrow Hoechst stained and labeled for Lin, SCA-1, CD117, and CD34. Lin^neg/SCA-1⁺/CD117⁺ cells (KLS) were divided into subsets of CD34 high, low, and negative intensity (A). Provided for comparison is a CD117/CD34 plot of the SP fraction from the same experiment with the same CD34 high, low, and negative gates. Dotted lines on both plots indicate the level of matched-isotype control. Most SP cells were CD34^low or CD34^neg in mice 8 to 12 weeks of age (B, upper row). In contrast, significant numbers of KLS⁺/CD34^high were only seen in the older (> 15 weeks) mice. Most interestingly, in all 3 older mice analyzed, the KLS⁺/CD34^neg subset was associated with the cells which most efficiently effluxed the Hoechst dye (B, lower panel).