Effects of synthetic retinoids on HL-60 and NB4 clonal proliferation. HL-60 and NB4 cells (2 × 10³ per plate) were cultured with various concentrations of retinoids in methylcellulose. Colonies (>40 cells) were counted after 10 days of incubation. Results are expressed as the percentage of clonal growth in retinoid-treated plates compared with the number of colonies in control plates not containing retinoids. Data represent mean ± standard deviation (SD) of triplicate cultures. This figure shows representative findings of three independent experiments, each of which had similar results.