For the remaining unresolved cases, possible pathogenic causes may include: 1) a second mutant allele in GJB2 may exist deeply in the introns or the non-coding regulatory regions uncovered by the targeted NGS; 2) mutation in a yet unknown deafness-associated gene may lead to the hearing loss in coordination with or independent to the GJB2 mutation; and 3) in some sporadic cases environmental factors may contribute to the hearing loss. Here, GJB2 is linked to deafness.