RA treatment reduces SK-N-SH neuroblastoma cell invasiveness. Invasiveness of NB cells is significantly reduced after 6 days RA treatment. The amount of invasive cells has been evaluated by means of a fluorescence assay (see Materials and Methods). Data are expressed as relative fluorescence units (RFU). Columns are means ± standard error; N = 4; double star = p < 0.01. Representative images of the cells on the bottom side of the insert filter show that all NB cells invading the matrix are DCX+ (red) in both control and RA treated samples. Nuclei are counterstained with DAPI (blue).