Cell development in short-term culture assays. ALDH^neg CD34⁺, ALDH^br CD34⁺, and ALDH^br CD34^neg cells were purified from lin^neg SSC^lo UCB, as depicted in Figure 1. Two hundred to 1000 purified cells were cultured on STO fibroblasts in the presence of IL-3, IL-7, and IL-15 (n = 10). Cultures initiated with ALDH^neg CD34⁺ cells contained higher percentages of CD56⁺ lymphoid progeny than did the ALDH^br CD34⁺ cells (compare panels A and B; see Table 4). Similarly, the ALDH^neg CD34⁺ cell fraction yielded lower percentages of CD13⁺ myeloid progeny (compare panels D and E). Only 2 cultures initiated with ALDH^br CD34^neg cells had growth significant enough to evaluate lineage development, and these gave rise to CD56⁺ progeny (C,F). The CD56⁺ progeny of ALDH^neg CD34⁺ (G-I) and ALDH^br CD34⁺ cells (data not shown) expressed other antigens consistent with NK cells. (J) Relative output of lymphoid cells was higher in cultures initiated with ALDH^neg CD34⁺ cells when compared with paired cultures initiated with ALDH^br CD34⁺ cells. (K) Conversely, the relative cell output of myeloid cells was higher in cultures initiated with ALDH^br CD34⁺ cells when compared with paired cultures initiated with ALDH^neg CD34⁺ cells. Estimations for relative cell output are described in “Materials and methods.” (J-K) Each data point represents an average from duplicate cultures.