Based on these phenotypic observations and our data indicating that TFAP2B regulates Pkhd1 and Cys1, we propose that Tfap2b, with Pkhd1 and Cys1, are components of a transcriptional network that modulates renal tubular cell functional differentiation, which when disrupted leads to an ARPKD-like phenotype. The gene discussed is CYS1; the disease is autosomal recessive polycystic kidney disease.