Until now, the relevance of these effects of cystin deficiency for human disease was unclear in the absence of ARPKD patients with variants in human CYS1. Here we present the first case of human ARPKD due to homozygosity for a CYS1 variant, in this case predicted to cause defective splicing. The gene discussed is CYS1; the disease is autosomal recessive polycystic kidney disease.