SLC26A4 and hearing loss disorder: The proportion of SLC26A4-associated hearing loss in Tuvinian patients (the Tyva Republic, Southern Siberia) is one of the highest in the world (28.2 %) and the vast majority of mutant SLC26A4 alleles are represented by three pathogenic variants c.919-2A>G, c.2027T>A and c.1545T>G (69.3, 17.5 and 8.0 %, respectively).