TTR and primary systemic amyloidosis: ATTR can result either from slow deposition of amyloid fibrils derived from wild‐type (non‐mutant) TTR in the elderly (senile systemic amyloidosis) or when mutations in the TTR gene encode variant protein, decreasing the stability of the TTR tetramer and promoting misfolding into amyloid fibrils (Rapezzi et al., 2010).