Circulating donor cells in NOD/SCID recipients. Nucleated PB cells were obtained 4 weeks after in utero transplantation and stained with antibodies to Ly5.1 (donor) cells and Ly5.2 (host) cells. The light scatter profile of host cells (gray) and the donor cells (black) is shown in (A). The frequency of Ly5.1 donor cells and Ly5.2 host cells was determined by flow cytometry (B). A control noninjected NOD/SCID mouse is shown in (C). The percentage of total cells that express either the donor or the host cell marker is indicated.