INVS and situs inversus: In humans, INVS was identified as the gene encoding Nephrocystin-2 (Nphp2) that is mutated in the recessive cystic kidney disease nephronophthisis type 2/infantile nephronophthisis [2], which, to a variable extent, is accompanied by situs inversus and other phenotypic traits of the inv−/− mouse and retinitis pigmentosa [10], [11].