Effect of retinoids on the growth of PLB-985 and NB4 cells. (A) PLB-985 cells were grown in the presence of 10⁻⁷ mol/L 9-cis-RA, 13-cis-RA, or all-trans-RA, and viable cells were counted at 1, 2, 4, and 7 days after the addition of retinoids. (B) PLB-985 and NB4 cells were grown in the presence of 10⁻⁷ mol/L 9-cis-RA or the selective ligands, and cell counts were made at days 1, 2, 3, 4, 5, and 7. Fresh medium and retinoids were added at day 3 and cell numbers were adjusted as described in Materials and Methods.