Isoform 1: FAD-dependent sulfhydryl oxidase that regenerates the redox-active disulfide bonds in CHCHD4/MIA40, a chaperone essential for disulfide bond formation and protein folding in the mitochondrial intermembrane space. The reduced form of CHCHD4/MIA40 forms a transient intermolecular disulfide bridge with GFER/ERV1, resulting in regeneration of the essential disulfide bonds in CHCHD4/MIA40, while GFER/ERV1 becomes re-oxidized by donating electrons to cytochrome c or molecular oxygen. {ECO:0000269|PubMed:19397338, ECO:0000269|PubMed:20593814, ECO:0000269|PubMed:21383138, ECO:0000269|PubMed:22224850, ECO:0000269|PubMed:23186364, ECO:0000269|PubMed:23676665}.; Isoform 2: May act as an autocrine hepatotrophic growth factor promoting liver regeneration.,Alternative sequence (1); Beta strand (1); Binding site (2); Chain (1); Disulfide bond (4); Domain (1); Erroneous initiation (4); Frameshift (1); Helix (7); Modified residue (1); Natural variant (2); Nucleotide binding (3); Sequence conflict (3),Ubiquitously expressed. Highest expression in the testis and liver and low expression in the muscle. {ECO:0000269|PubMed:19409522}.,H2O2 [CHEBI:16240]; O2 [CHEBI:15379]; R'C(R)S-S(R)CR' [CHEBI:17412]; R'C(R)SH [CHEBI:16520]; FAD [CHEBI:57692],H2O2 [CHEBI:16240]; O2 [CHEBI:15379]; R'C(R)S-S(R)CR' [CHEBI:17412]; R'C(R)SH [CHEBI:16520],FAD [CHEBI:57692],Myopathy, mitochondrial progressive, with congenital cataract, hearing loss and developmental delay (MPMCHD) [MIM:613076]: A disease characterized by progressive myopathy and partial combined respiratory-chain deficiency, congenital cataract, sensorineural hearing loss, and developmental delay. {ECO:0000269|PubMed:19409522, ECO:0000269|PubMed:20593814}. Note=The disease is caused by mutations affecting the gene represented in this entry.