Four-color flow cytometric analysis of TdT⁺ precursors with respect to expression of CD79a and CD19. BM MNCs of a healthy girl (2 years and 7 months old) were prepared from a specimen collected at BM donation for her diseased sister. Cells were stained with CD19, CD79a, and TdT, together with either CD10, CD34, CD33, or CD7. Isotype controls were also included in the experiment. First, 30,000 total events of each individual sample were acquired. Next, live gates were set in parameter correlations engaging T-SSC, TdT, and CD79a, and approximately 10,000 TdT⁺CD79a⁺ events were acquired from the same material. Dot plots of relevant parameter correlations (logarithmic scale, except for T-SSC: linear scale) show aspects of total MNCs in row A (large plots). The small inserted plots in A visualize the respective isotype controls for each antigen. The large dot plots of rows B and C show gated events, whereas the small inserted plots display the same marker correlations in ungated mode. Only TdT⁺ precursors are shown in colors: the subset characterized by coexpression of CD79a and CD19 is depicted in green, whereas CD79a⁺ precursors lacking CD19 are painted red. The few TdT⁺ cells that lack both CD79a and CD19 are painted black (see A). All other TdT⁻ cells appear grey. Note that CD19⁻CD79a⁺TdT⁺cells (red) are CD34⁺, display slightly elevated TdT expression as well as T-SSC properties, and express CD10 at a lower level compared with CD19⁺ precursors (green). Weak CD33 positivity can also be seen on some cells of the former subset, whereas CD7 expression is very rare in either subset of this sample.