In adults over 50 years of age, Fanconi syndrome is frequently related to the urinary secretion of a monoclonal immunoglobulin (Ig) light chain (LC), almost always of the kappa isotype. Prevalence is unknown but around 100 cases have been described in the literature so far. Onset occurs during adulthood: bone pain related to osteomalacia secondary to hypophosphataemia and progressive chronic renal failure are the usual manifestations. These manifestations may precede the diagnosis of a slowly progressive plasma cell disorder by several years. In most cases, the monoclonal kappa light chain is restricted to the V kappa-1 subgroup and bears non-polar or hydrophobic mutations in the variable domain, which induce resistance to cathepsin proteolysis in the proximal tubular cells and promote crystallisation of the variable domain within the endolysosomal compartment. [from SNOMEDCT_US]