HCA expression by human BM stromal cells in primary cultures derived from (A) total ABM cells: the cells were stained with MoAbs against rat HCA (11-59), HCA, Thy-1, CD34, and CD6, and with anti-CD45 to discriminate hematopoietic cells; and (B) Stro-1⁺ stromal progenitors sorted from ABM. For each antibody the fluorescence histogram is presented along with that of an irrelevant antibody of the same isotype (heavy line).