TTC21B and kidney disorder: Among the patients who developed kidney failure without a known etiology, we confirmed the monogenetic kidney disorders in 11 known disease-causative genes, including PAX2 (n = 4), COQ8B (n = 4), NPHP1 (n = 3), WT1 (n = 2), COL4A5 (n = 1), COL4A4 (n = 1), ANKS6 (n = 1), TTC21B (n = 1), SLC34A1 (n = 1), and SALL1 (n = 1).