Expression of CD26 and CXCR4. CD26 cell surface expression was measured by flow cytometry using the following fluorochrome-conjugated monoclonal antibodies simultaneously: CD26-FITC, CXCR4-PE, Sca-1-PECy5.5, and c-kit-APC. (A) Representative isotype control is shown. (B) CD26 is expressed on 73% of Sca-1⁺c-kit⁺lin^— cells. Simultaneous examination of CXCR4 expression in these cells reveals that the majority of CD26⁺ and CD26^— cells express CXCR4. Sca-1⁺c-kit⁺lin^— cells have one distinct population of cells with respect to CD26 expression, of which 73% fall in the CD26⁺ category as compared with the isotype control. (C) CD26 is expressed on 75% of Sca-1⁺c-kit^—lin^— cells, and of those the majority are CXCR4⁺. Unlike Sca-1⁺c-kit⁺lin^— cells, Sca-1⁺c-kit^—lin^— cells have 2 distinct CD26⁺ and CD26^— populations. Data obtained from 6 independent mBM samples indicate that a significant percentage of normal HSCs/HPCs from mBM express CD26. Representative sample Sca-1⁺c-kit⁺lin^— (B) and Sca-1⁺c-kit^—lin^— (C) dot plots are shown.