Oxygen sensing across kingdoms The ability to sense and respond to changes in oxygen levels is critical for most forms of life. To date, mechanistic studies of this process in mammals have focused on the oxygen-sensitive stability of a transcription factor called hypoxia-inducible factor. Masson et al. discovered an enzymatic oxygen sensor in humans that is functionally identical to plant cysteine oxidases, enzymes that control responses to hypoxia in plants. The human and plant enzymes convert the N-terminal cysteine in substrate proteins to cysteine sulfinic acid, a modification that ultimately targets the proteins for degradation. Oxygen sensing is impaired in many human diseases, and further study of the human enzyme could help in the development of strategies for therapeutic intervention. Science , this issue p. 65