Until 1994, several spontaneous mutations in mouse lines had been described resembling ARPKD in humans, namely cpk (congenital polycystic kidneys) [65] mapped in 1991 to mouse chromosome 12 [66], bpk (BALB/c polycystic kidneys) [67], pcy (polycystic kidneys) [68] mapped in 1991 to mouse chromosome 9 [69] and jck (juvenile cystic kidneys) [70] and others (reviewed in [71]), but none of the disease-causing genes had been identified at that time, making it impossible to study the pathogenesis of PKD in more detail. The gene discussed is BTK; the disease is polycystic kidney disease.