Clonal inhibition of HL-60 and NB4 cells after pulse-exposure (3 days) to retinoids. HL-60 and NB4 cells were exposed in liquid culture to 10^-11 to 10^-7 mol/L of either ATRA, Retinoids B, C, D, E (• for HL-60, ○ for NB4, A through E), or a combination of 10^-11 to 10^-7mol/L of either Retinoids D or E plus 10^-9 mol/L ATRA (• for HL-60, ○ for NB4, D and E), washed, plated in methylcellulose, and the resulting colonies counted. Each point represents a mean ± SD of triplicate dishes. This figure shows representative findings of three independent experiments, each of which had similar results.