GJB2 and sensorineural hearing loss disorder: The level of mitochondrial 12S rRNA A1555G heteroplasmy tested by NGS in the 100 cases with normal hearing and 100 patients with severe sensorineural hearing loss negative for GJB2, SLC26A4, and mtDNA 12S rRNA was less than 1% and did not differ significantly (P > 0.05) between these two groups.