While homozygous PKD1 mutations appear to be incompatible with life [39], biallelic hypomorphic PKD1 mutations can cause severe ARPKD-like forms [40, 41], and the combination of a classical and an ultra-low penetrant PKD1 mutation can also cause severe, early ADPKD [42]. Here, PKD1 is linked to autosomal dominant polycystic kidney disease.