Down regulation of CDK6 promoter activity by I3C and N-alkoxy I3C derivatives. MCF-7 cells stably transfected with the −920 bp CDK6-luciferase reporter plasmid were treated with 200 μM I3C, 100 μM tryptophol, 10 μM N-methoxy I3C, 5 μM N-ethoxy I3C, 1 μM N-propoxy I3C, or 0.5 μM N-butoxy I3C, or with the vehicle DMSO control for 48 hours and assayed for luciferase activity as described in the Methods and Materials. The relative light units per μg protein are representative of three independent experiments of triplicate samples, and the error bars indicate the standard deviation.