In this regard, the nuclear localization of FKBP51 observed in most cancer cells (rather than the cytoplasmic/mitochondrial localization seen in normal cells) [21] is perhaps the direct consequence of the high production of ROS in these cells, a stimulus that is tightly linked to the enhanced catalytic activity of protein kinases [47,48,49,50]. This evidence concerns the gene FKBP4 and cancer.