Surface c-kit expression identifies Lin⁻ BM populations enriched for TdT⁺ cells. Lin⁻BM, representing approximately 2% of BM mononuclear cells, was sorted from whole BM depleted of Lin⁺ cells by magnetic separation (A), and costained for intranuclear TdT and surface c-kit as described in Materials and Methods. Within costained Lin⁻BM, c-kit⁻, c-kit^Lo, and c-kit^Hipopulations were identified (B). Lin⁻ BM was gated for TdT⁺ cells (C). (D) shows expression of c-kit in gated Lin⁻ TdT⁺ cells. The frequencies of these subsets in BM are shown in Table 1.