The Pfr state passes on a signal to other biological systems in the cell, such as the mechanisms responsible for gene expression. Although this mechanism is almost certainly a biochemical process, it is still the subject of much debate. It is known that although phytochromes are synthesized in the cytosol and the Pr form is localized there, the Pfr form, when generated by light illumination, is translocated to the cell nucleus. This implies a role of phytochrome in controlling gene expression, and many genes are known to be regulated by phytochrome, but the exact mechanism has still to be fully discovered. It has been proposed that phytochrome, in the Pfr form, may act as a kinase, and it has been demonstrated that phytochrome in the Pfr form can interact directly with transcription factors.