tokens: Comparison of the relative potency of the retinoids using the nitroblue tetrazolium reduction test showed effective concentrations required to differentiate 50 % of cells in 72 h as follows : ATRA , 15.8 +/- 1.7 nM ; 4-oxo-RA , 38.3 +/- 1.3 nM ; 18-OH-RA , 55.5 +/- 1.8 nM ; 4-OH-RA , 79.8 +/- 1.8 nM ; and 5 , 6-epoxy-RA , 99.5 +/- 1.5 nM .