B2M and amyloidosis: The approach for the subtyping of amyloidosis has evolved over the last 30 years, moving from histochemistry to more specific immunohistochemistry of the major precursor proteins, such as serum amyloid A (SAA), transthyretin (TTR), immunoglobulin kappa light chain (IGK), immunoglobulin lambda light chain (IGL), and beta-2-microglobulin (B2M).