PKHD1 and cyst: The second hit model does not explain cyst formation in autosomal recessive PKD, in which patients harbor a trans‐heterozygous mutation in PKHD1. Nor can it explain the rare patients who are trans‐heterozygous for an incompletely penetrant PKD1 allele and a pathogenic PKD1 allele.19 In these cases, patients already have both alleles mutated and still exhibit focal cyst formation.