**3. Protein phosphatases**

As compared the large number of kinases, the phosphatases comprise less than 1% of the human coding genes [43]. The phosphatase enzymes dephosphorylate target proteins at the serine or threonine residues (protein serine/threonine phosphatases), while some act on the tyrosine residues (protein tyrosine phosphatases) or both tyrosine and serine/threonine residues (dual-specificity phosphatases). The protein serine/threonine phosphatases are divided into three structurally related groups while all members of protein tyrosine phosphatases and dual-specificity phosphatases belong to one structurally related class. The, atypical protein phosphatases constitute a separate group, with structural features different from the other types [44]. In contrast to the protein kinases, research has identified far fewer protein phosphatases, which have an important role in the auditory system (**Table 1**). These enzymes are important for disparate developmental processes, and the targeted deletions of the pertinent genes in mice have revealed their contributions to the development of ear and maintenance of hearing. In some cases, although the phosphatase itself may not have been directly implicated yet in a human hearing loss disorder, variants in their substrate or docking proteins do cause deafness [45].
