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Table 2. Expression of Jl and<br />

Source of clones No. clones % Clones positive for<br />

TdT by A-MuLV -transfortested<br />

marker med cells a<br />

Jl<br />

TdT<br />

Neonatalliver 4 100 100<br />

Neonatal marrow 3 67 100<br />

17-19 d fetalliver 16 75 75) of celllines isolated frorn<br />

adult bone marrow. In addition, clones from<br />

neonatal animals are similar to the clones from<br />

adult tissue with respect to frequency of!l and<br />

TdT positive cells. Thus, changes in the frequencies<br />

of !l and TdT expression among fetal<br />

liver-derived cell lines probably reflect changes<br />

in the predominant target cell population<br />

during ontogeny. This shift may result from the<br />

different proportions of certain types of lymphoid<br />

precursors existing in rapidly cyc1ing<br />

states at different times during gestation.<br />

Acknowledgments<br />

This work was supported by Public Health Service<br />

Grant CA-24220 and Program Project grant<br />

CA-24530 from the National Cancer Institute. N.R.<br />

is a recipient of an American Cancer Society<br />

Research Scholar Award (Massachusetts Division).<br />

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470

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