In people with this disease, small fibres of TTR protein clump together to make deposits called amyloid. Amyloid can build up around or within the nerves, heart and other organs, stopping them from working properly. Eplontersen is specifically indicated for use when the disease is causing polyneuropathy, which is damage to multiple nerves outside of the brain and central nervous system, resulting in pain, discomfort, progressive weakness and loss of sensation in the legs and arms, and mobility difficulties.