The flavin-dependent sulfhydryl oxidase from chicken egg white catalyzes the oxidation of sulfhydryl groups to disulfides with the reduction of oxygen to hydrogen peroxide. Reduced proteins are the preferred thiol substrates of this secreted enzyme. The egg white oxidase shows an average 64% identity (from randomly distributed peptides comprising more than 30% of the protein sequence) to a human protein, Quiescin Q6, involved in growth regulation. Q6 is strongly expressed when fibroblasts enter reversible quiescence (Coppock, D. L., Cina-Poppe, D., Gilleran, S. (1998) Genomics 54, 460-468). A peptide antibody against Q6 cross-reacts with both the egg white enzyme and a flavin-linked sulfhydryl oxidase isolated from bovine semen. Sequence analyses show that the egg white oxidase joins human Q6, bone-derived growth factor, GEC-3 from guinea pig, and homologs found in a range of multicellular organisms as a member of a new protein family. These proteins are formed from the fusion of thioredoxin and ERV motifs. In contrast, the flavin-linked sulfhydryl oxidase from Aspergillus niger is related to the pyridine nucleotide-dependent disulfide oxidoreductases, and shows no detectable sequence similarity to this newly recognized protein family.