TTC21B and Bardet-Biedl syndrome: Triallelic inheritance has been proven in BBS, while oligogenic inheritance has been observed in clinical cases for genes, such as NPHP 1, NPHP 5, NPHP 6, NPHP 8, NPHP 9, NPHP 11, and Tetratricopeptide Repeat Domain 21B (TTC21B), etc. [54,55].