TTC21B and ciliopathy: All ciliopathy subgroups share the gene TTC21B, and both primary and secondary ciliopathies share the genes IFT140, BBS1, BBS2, ARL6, TTC8, CEP290, IFT172, C8orf37, IFT43, GLI3, INPP5E, OFD1, SUFU, ARL3, CC2D1A, GUCY2D, RPE65, RPGRIP1, CRX, CRB1, IMPDH1, RDH12, TULP1, PRPH2, CEP55, CEP83, DCDC2 and DYNC2I2.