PLK4 and retinitis pigmentosa: This identified biallelic variants in two cilia or centrosomal genes which have been linked to other OMIM diseases but not PCD (Table 3; cases 18 + 19): IFT140, which has been reported in isolated retinitis pigmentosa (RP) [27, 28] and syndromic RP [29] and several short rib polydactyly syndromes [30–32] and PLK4 which has been associated with microcephaly and chorioretinopathy [33].