Immunofluorescence staining with kappa light chain and lambda light chain antibodies shows negative kappa in casts that are lambda positive in serial section. Monoclonal light chain staining in atypical casts is characteristic of light chain cast nephropathy (LCCN). In this case, a monoclonal lambda clone is responsible though kappa light chains are the more common cause with a 3 to 1 predominance. These casts are not only tubulotoxic but are also obstructive.