PKD2 and cyst: Genetic studies support a threshold model in which cyst formation is triggered by reduced functional polycystin dosage below a critical threshold within individual tubular epithelial cells, due to germline and somatic PKD1 and/or PKD2 mutations, plus mutations of genes (e.g., SEC63, SEC61B, GANAB, PRKCSH, DNAJB11, ALG8, and ALG9) in the endoplasmic reticulum protein biosynthetic pathway, or somatic mosaicism (the presence of two genetically distinct cell populations within one individual resulting from a somatic mutation during embryogenesis) [5].