Risk of progression of monoclonal gammopathy of undetermined significance (MGUS) to multiple myeloma based on the serum free light chain ratio. The upper curve illustrates risk of progression of monoclonal gammopathy of undetermined significance in patients with an abnormal serum kappa/lambda free light chain ratio (< 0.26 or > 1.65). The lower curve illustrates the risk of progression in patients with a normal ratio. This figure was originally published in Blood and is reproduced with permission