A novel protein encoded by the C210RF2 gene in chromosomal locus 21q22.3 was characterized by immunochemistry. This chromosomal region is known to contain genes for human diseases such as non-syndromic autosomal recessive deafness (DFNB8/10) and autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED). Polyclonal murine antisera were produced against the multivalent peptides deduced from the amino acid sequence of the polypeptide. Immunological reactivity of the obtained antisera was tested with primary cells or established cell lines. On western blotting, the polyclonal sera recognized a single protein product of 25 Kd expressed in cell lines of epithelial and lymphoid origin. Subsequent immunochemistry of several human tissues indicated the ubiquitous expression of the protein. Immunofluorescence studies and co-staining with a mitochondrial-specific dye suggest the subcellular localization of the protein to mitochondria. Mitochondrial localization is also predicted by computer analysis of the polypeptide sequence. As deafness is known to be caused in some instances by defects in mitochondrial function, C210RF2 is a plausible candidate gene for DFNB8/10.