RBP4 and COVID-19: Patients with ATTR amyloidosis are particularly vulnerable to COVID-19 [80]; retinoic acid receptor responder 2 (CRABP-II), involved in transporting retinoic acid (RA) from the cytosol to the nucleus, enhancing cell transcriptional activity [53]; retinol-binding protein 4 (RBP4), which transports retinol from liver stores to peripheral tissues [54].