Direct contact with stromal ligands, IL-2, IL-7, SCF, and hydrocortisone induces NK cell progenitor expansion. Fresh CD34⁺/Lin⁻/DR⁻ cells were plated into functional LDAs under long-term NK cell culture conditions with IL-2 alone to determine the day 0 cloning frequency, which for each experiment was assigned the day 0 baseline (white bars). The absolute cloning frequency for day 0 was 0.04% ± 0.01%. NK cell progenitor expansion was evaluated by plating 20,000 fresh CD34⁺/Lin⁻/DR⁻ cells in direct contact with stroma in LTC medium with (black bars) or without (hatched bars) hydrocortisone. After 14 days, progeny of these cultures were trypsinized and plated into LDA in NK medium. The cloning frequency was determined based on the number of CD34⁺/Lin⁻/DR⁻ cells initially inoculated into culture and is reported by experiment compared with the same donor's day 0 LDA.