Despite the much more severe phenotype of ARPKD with progression to end-stage renal disease of the majority of the patients within the first two decades of life (9) [as compared to 58.1 years (PKD1) and 79.9 years (PKD2) in ADPKD (26)], the large individual phenotypic variability and the 40-fold lower incidence of ARPKD relative to ADPKD have so far precluded the development of clinical trial programs to evaluate the usefulness of compounds interfering with cyst growth. The gene discussed is PKD1; the disease is autosomal recessive polycystic kidney disease.