Familial juvenile nephronophthisis (FJN): A childhood genetic kidney disease in which there is progressive symmetrical destruction of the kidneys involving both the tubules and glomeruli, characteristically resulting in anemia, polyuria, polydipsia, isosthenuria (decreased ability to concentrate the urine), progressive renal failure and death in uremia. Hypertension and proteinuria are conspicuous by their absence. The chronic kidney failure affects growth and leads to short stature. The age at death ranges from around 4 to 15 years.

The disease is also called nephronophthisis 1 (NPH1) and autosomal recessive medullary cystic kidney disease. It is inherited as an autosomal recessive trait. It was first described by Guido Fanconi and his colleagues in 1951.

The gene for the disease is on chromosome 2 in region 2q13 and is designated NPHP1. It codes for a protein dubbed nephrocystin. About 70% of cases are caused by large deletions in the 2q13 region in both of the child's number 2 chromosomes. The deletions can be readily detected by a molecular genetic test using the PCR (polymerase chain reaction).