Although a variety of in vitro functional studies of Cx26, Cx30, and Cx43 mutations have been carried out and various possible pathogenic changes (described in the Section 3 of this review) have been revealed, it remains unclear why some Cx26 and Cx30 mutations cause deafness without skin diseases in vivo but others induce syndromic deafness accompanied with skin changes. Here, GJA1 is linked to deafness.