**2.3 Protein tyrosine kinases, nonreceptor type**

So far variants in two different genes encoding nonreceptor protein tyrosine kinases, ABL1 and BTK, have been reported to cause hearing loss in some patients with different syndromes. Variants of *ABL1* cause a syndromic disorder (**Table 1**) in which subsets of patients have outer ear abnormalities and hearing loss [40]. Recently, variants were reported in patients with a phenotype termed as *ABL1* malformation syndrome, and it was shown that hearing loss in the patients occurred due to the increased tyrosine kinase activity of the protein [41]. Variants of *BTK* have been reported to cause otitis media and hearing loss in a few patients with agammaglobulinemia, X-linked 1, a disorder of B-lymphocyte maturation [42].
