CD34+CD38+ intermediate hematopoietic progenitors express cell surface receptors necessary for HIV-1 entry. (A) Within the CD34+ HPC population, CD38 coexpression indicates a more mature phenotype and includes the CMP, MEP, and GMP. (B) CD34+ cells were enriched from fetal liver and assayed by flow cytometry for the presence of HIV entry receptors in conjunction with definitive surface markers of intermediate hematopoietic progenitors. CD34+CD38+ double-positive cells were gated (left) to select the CMP, GMP, and MEP based on CD123 and CD45RA expression (right). (C) Each population of intermediate hematopoietic progenitor was examined for CD4 and CCR5 expression by flow cytometry. (D) These same populations were also examined for CD4 and CXCR4 expression by flow cytometry. LT-HSC, long-term hematopoietic stem cell; MPP, multipotent progenitor; ST-HSC, short-term hematopoietic stem cell.