Abstract: A modified igNAR peptide sequence derived from a wild-type igNAR peptide sequence is diversified by mutating the amino acid sequence at 50% or more of the amino acids in the CDR3 loop region and optionally at 50% or more of the amino acids in the CDR3 loop region. The modified igNAR peptide may have the sequence of SEQ ID NO: 8, 10 or 50 to 85. The modified igNAR peptides have binding activity against albumin protein sequences, such as human serum albumin. These modified igNAR peptides may have utility in extending the in vivo half-life of biological molecules e.g. therapeutic agents, and so may be used in medicine.