Stable transfection of constructs containing the entire mouse HO-1 gene and various portions of the 5'-flanking region into rat C6 glioma cells and simultaneous, quantitative analysis of the mouse and rat HO-1 mRNAs indicate that distal 5' sequences, between positions -3.5 and -12.5 kbp, are required for induction of mouse HO-1 gene transcription by both heme and heavy metals.