Signal transduction. How do cells sense oxygen?

How do organisms sense the amount of oxygen in the environment and respond appropriately when the amount of oxygen decreases (a condition called hypoxia)? In their Perspective, Zhu and Bunn discuss new findings (Ivan et al., Jaakkola et al.) that reveal how the HIF transcription factor, which switches on a group of hypoxia-response proteins, is itself regulated by changes in oxygen tension. The authors are in the Hematology Division of the Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Boston, MA 02115, USA. E-mail: zhu@calvin.bwh.harvard.edu, bunn@calvin.bwh.harvard.edu