HY1 (HEME OXYGENASE 1) is a plastid heme oxygenase necessary for phytochrome chromophore biosynthesis and for coupling the expression of some nuclear genes to the functional state of the chloroplast. Alternative names: HEME OXYGENASE 1,HEME OXYGENASE 1,GENOMES UNCOUPLED 2,HO1,ARABIDOPSIS THALIANA HEME OXYGENASE 1,HY6,HY1,ATHO1,HEME OXYGENASE 6,GUN2, REVERSAL OF THE DET PHENOTYPE 4.