Clonogenic lymphoid progenitors express CD45RA. Affinity-column–separated CD34⁺ normal bone marrow cells were stained with PE-conjugated CD45RA and a mixture of FITC-conjugated lineage-associated differentiation markers (CD10, CD19, CD20, CD2, CD3, CD4, and CD15⁻ lin). The subpopulations sorted for lymphoid progenitor colony assay are indicated by boxes. The small CD45RA⁺⁺/lin⁺ subpopulation (upper right) was sorted and found not to contain CFU-TdT. The cloning efficiency and proportion of total CFU-TdT within each sorted population, calculated as for Table 1, is indicated at right. Differences in colonies/10,000 cells and % colonies in the fraction between CD45RA⁺/lin⁻v CD45RA⁻/lin⁻ or CD45RA⁺/lin⁺ were significant (P < .05 by paired t-test). Errors are ± SE as an estimate of variation of means of three replicate wells in three to four separate experiments. Most lymphoid progenitor colonies originate from the CD45RA⁺/lin⁻ fraction, and virtually none from the CD45RA⁻ fractions. Similarly, only 0.05% (±0.1% SD, n = 4) of CD45RA⁻/lin⁻ cells were TdT⁺ while 19.3% (±5.5% SD, n = 3) of CD45RA⁺/lin⁻ and 55.7% (±22.1% SD, n = 3) of CD45RA⁺/lin⁺ cells were TdT⁺. In a separate experiment, 94% of the CD45RA⁻/lin⁻ fraction was shown by staining with CD34 antibody to be CD34⁺.