SLC26A4 and Pendred syndrome: Functional testing can represent a strong criterion for pathogenicity assignment or exclusion and, concerning pendrin variants, was made by the quantification of radiolabeled chloride or iodide fluxes in various heterologous expression systems, including Xenopus laevis oocytes, insect Sf9 cells, mammalian and human cells [17,18,19,20,21,22], or even in primary cultures of thyrocytes from a patient with Pendred syndrome [23].