NK development in secondary LTC assays. Secondary NK cultures were established from primary LTCs that had been originally initiated with 50 to 500 cells per culture (n = 6). (A,C) Secondary cultures were analyzed for their content of CD56⁺ NK cells and CD13⁺ myeloid cells. (B,D) To represent the entire data set, the relative percentages of myeloid and lymphoid progeny were plotted for each culture, where each data point represents the average of duplicate cultures. (A-B) High percentages of CD56⁺ cells (more than 50%) were present in secondary cultures derived from primary LTCs originally established with lin^neg SSC^lo ALDH^br CD34⁺ cells. The single exception was from primary cultures that had been initiated with 50 cells/well. (C-D) In contrast, of the cultures established with lin^neg SSC^lo ALDH^neg CD34⁺ cells, 2 secondary cultures gave rise to predominantly CD13⁺ myeloid progeny, and 1 secondary culture had no evident growth.