It has been repeatedly shown that patients with biallelic SLC26A4 variants (M2) have an earlier age of HL onset, their HL is more severe, and they are more often diagnosed with Pendred syndrome than patients with a monoallelic SLC26A4 variant and CEVA (M1 or M1 + CEVA) or without pathogenic variants in the SLC26A4 gene (M0) [26,58,59]. This evidence concerns the gene SLC26A4 and Pendred syndrome.