The haem oxygenase (HO) enzyme catalyses the oxidation of haem to biliverdin IX alpha, CO and Fe(2+), and performs a wide variety of roles in Nature, including degradation of haem from haemoglobin, iron acquisition and phycobilin biosynthesis. In plants, HOs are required for the synthesis of the chromophore of the phytochrome family of photoreceptors. There are four HO genes in the Arabidopsis genome. Analysis of a mutant deficient in HO1 (the hy1 mutant) has demonstrated that this plastid-localized protein is the major HO in the phytochrome chromophore synthesis pathway. HO2 may also have a minor role in this pathway, but our understanding of the divergent roles of this small gene family is still far from complete.