CYS1 and autosomal recessive polycystic kidney disease: Furthermore, from a clinical perspective, we speculate that among the approximately 20% of patients with ARPKD who lack mutations in PKHD1, DZIP1 or CYS1 coding sequences, there may be patients for whom disease is due to sequence variants in the TFAP2B binding sites of PKHD1 or CYS1. Alternatively, these patients may carry novel mutations directly affecting TFAP2B-regulated genes that are expressed in renal collecting duct epithelia.