RARA and Cowden disease: To examine whether RA/RAR activity in CD-derived ARPKD renal cystic cells is abnormal, we compared the endogenous RA/RAR activity in primary cultures of embryonic (foetal 24 weeks), perinatal (newborn) and paediatric (18 months old) ARPKD renal cystic cells and their age-matched normal human CD cells.