Coexpression of HIV-1 X4 coreceptors on BM, UCB, and MPB CD34⁺ cells. Flow cytometric analysis of purified CD34⁺ cells labeled with isotype control PE/TC (A) or anti-CD4 PE and anti-CXCR4 TC (B, bone marrow; C, umbilical cord blood; D, G-CSF–mobilized peripheral blood).