Restriction to the NK lineage occurs as Lin⁻ c-kit^Lo cells acquire the CD122 receptor for IL-15.

Lin⁻ c-kit^Lo cells were first cultured for 3 days with SCF, FL, and IL-7. They were then stained, analyzed, and sorted into 4 categories based on the acquisition of CD45R/B220 or CD122 (A). Each subset was then returned to culture along with SCF, FL, IL-7, and IL-15 for an additional 5 days. Finally, the recovered cells were analyzed by flow cytometry to assess the generation of B-lineage (CD19⁺) or NK-lineage (DX5⁺) lymphocytes (B). Absolute yields per input cells were calculated following primary and secondary cultures and are presented in Table 2. A similar experimental design was used to determine which of the 4 subsets present at the end of the first culture interval were sensitive to estrogen. The results are presented as yields of NK- or B-lineage cells per precursor placed in the secondary cultures (C).