The functional upstream domain (FUD, also known as pUR4), is a 6-kD peptide derived from Streptococcus pyogenes F1 adhesin that has high binding affinity to the N-terminus of FN and shows promise as a molecular probe to target FN in the context of pulmonary fibrosis [19, 26, 27]. The gene discussed is FN1; the disease is pulmonary fibrosis.