PKD2 and cyst: These include hypomorphic or incompletely penetrant alleles (50); PKD1 or PKD2 homozygosity (47); compound heterozygosity (51); trans-heterozygosity (52); somatic and germ-line mosaicism (53); epigenetic regulators (54–57); genetic modifiers (58); co-inheritance of a PKD1 or PKD2 mutation and an additional cyst-causing gene such as HNF1β (59, 60) or the tuberous sclerosis 2 gene (61); and alternative splicing of Pkhd1 that produces transcripts with distinct expression patterns and function (62).