Dose- and time-dependent effects of 1,25(OH)₂D₃ and/or all-trans RA on cellular proliferation of UF-1 cells. RA-resistant UF-1 cells were cultured in 96-well plates with various concentrations (10⁻¹⁰ to 10⁻⁷ mol/L) of 1,25(OH)₂D₃ for 4 days (A) or for different times (0-96 hours) with the indicated compounds (B), and then MTT incorporation was measured. Absorbance at 570 nm (OD₅₇₀) was recorded using an enzyme-linked immunosorbent assay plate reader. Results are presented as the mean of triplicate experiments; the SD was within 10% of the mean.