TTC21B and kidney disorder: There was one patient with TTC21B single heterozygous mutation in group A and three patients with compound heterozygous or homozygous mutations in group B. Given that TTC21B has been identified as a causative gene for familial FSGS, we performed a literature review to investigate potential correlations between proteinuria severity and mutation zygosity (single heterozygous vs. biallelic) in TTC21B-associated nephropathy.