B2M and primary systemic amyloidosis: The β2m variants studied here in vitro have different effects in vivo, with distinct human pathology associated with each: an iatrogenic joint arthropathy for β2m-ΔN6; a familial, systemic amyloidosis for β2m-D76N; and an iatrogenic deposition in the tongue and salivary glands for β2m-V27M.