Structural studies of the FIH enzyme revealed an interesting fold, identical to a domain called Jumonji C (JmjC) . JmjC enzymes are protein demethylases, and a number of them possess histone demethylase activity . As such, these enzymes have the potential of sensing and being controlled by oxygen levels in the cell. Changes in histone methylation have been described in hypoxia in a number of cellular systems . One of the JmjC enzymes, lysine (K)-specific demethylase (KDM)4E, has been analysed in terms of its oxygen dependency, revealing a similar response to oxygen to that of PHDs . Interestingly, some of the JmjC enzymes have been shown to be induced in hypoxia, and some in a HIF-dependent manner (reviewed in ). These include KDM3A, KDM4B and C, and KDM5C .