Flow cytometric analysis of β-gal expression intransgenic bone marrow populations. The lineage of β-gal⁺ cells was determined by simultaneous analysis for the indicated lineage-specific surface markers (see the Materials and Methods): (A) Mac1, a myeloid cell marker; (B) the erythroid marker Ter119; (C) B220, a B-lymphoid marker; and (D) IgM and (E) IgD, markers of mature B lymphocytes. Less than 1% of lineage marker-positive cells in the negative control bone marrow were β-gal⁺.