Since tasquinimod binds to both S100A9 and HDAC4 proteins with high affinities and since S100A9 has both an extracellular and intracellular location [24] while HDAC4 in prostate cancer cells is in the cell nucleus [12], it is important to evaluate whether tasquinimod is capable of entering cells or whether it only associates with extracellular plasma membrane components. This evidence concerns the gene S100A9 and prostate cancer.