TTC21B and focal segmental glomerulosclerosis: Both causal and a possible modifier of multiple ciliopathy genes. including BBS1, BBS2, BBS4, MKKS, BBS7, BBS10, BBS12, NPHP4, CC2D2A, and TMEM216 [51];TTC21B is a possible modifier in patients with FSGS and collagen type 4 gene variants (COL4A3 and COL4A5) [52].