Chronic granulomatous disease (CGD) is a rare inherited disorder associated with a profound predisposition to infection due to the lack of a microbicidal oxidase system in the phagocytes of these patients¹. This syndrome is most commonly inherited through a defect on the X chromosome and the only clearly defined component of the oxidase system, the very unusual cytochrome b (b₋₂₄₅), has been shown to be missing from the cells of these patients^2,3. This cytochrome is a heterodimer composed of an α-chain of relative molecular mass (M_r) 23,000 (23K) and a 76–92K β-chain; neither are detectable in neutrophils from X-linked CGD subjects. The defective X-CGD gene has recently been cloned⁴ by 'reverse genetics'⁵ but the protein predicted from the proposed complementary DNA sequence was not identified. We have purified the β-chain of the cytochrome and sequenced 43 amino acids from the N terminus. Almost complete homology was obtained between this sequence and that of the complementary nucleotides 19–147 of the sequence of the X-CGD gene, originally designated as a non-coding region.