The missing piece in the biochemistry of haem (heme) is reported in this week’s spectroscopyNOW. Japanese researchers have used x-ray diffraction to determine the crystal structure of the enzyme human indoleamine 2,3-dioxygenase (IDO). This crucial enzyme splits the pyrrole ring of the amino acid L-tryptophan and incorporates both atoms of a molecule of oxygen, an essential step in dozens of metabolic reactions. The discovery could have implications for other studies involving this enzyme and medical problems with which it is associated.