Expression of lymphoid lineage–associated genes and chemokine receptors in FB and FL subpopulations. The mRNA was prepared from IL-7R^– and IL-7R⁺ cells in FB and FL from 12-dpc fetuses, mRNA from whole 18-dpc FT cells, and adult bone marrow cells (BM) was used as controls. Samples equivalent to 300 cells were analyzed by RT-PCR.