In summary, these studies have demonstrated the following: 1) plasma TTR levels are significantly decreased in Portuguese patients with FAP; 2) FAP-TTR is indistinguishable from normal TTR with regard to a wide range of physiochemical properties; 3) RBP levels and vitamin A transport appear to be normal in FAP patients; 4) an abnormal TTR with a methionine for valine substitution at position 30 is found as amyloid protein in tissues of patients with FAP; and 5) this abnormal TTR is present in small amounts in plasma of patients with FAP. Based on these observations, we postulate that this abnormal form of TTR selectively deposits in tissues as the amyloid characteristic of the disease.