The present invention provides a mutant streptavidin subunit which comprises one or more amino acid substitutions compared to a wildtype streptavidin subunit at any one of residue positions equivalent to positions (50, 51, 52, 53) and (54) of SEQ ID NO. 2 and wherein amino acid residues at positions equivalent to positions (23, 27, 43, 45, 49, 79, 88, 90, 92, 108, 110) and (128) of SEQ ID NO. 2, in said mutant streptavidin subunit, are wildtype, wherein (i) when said mutant streptavidin subunit unit is comprised in streptavidin, said streptavidin has a lower off rate for biotin or for a biotin conjugate than wildtype streptavidin or (ii) when said mutant streptavidin subunit is in monomelic form said mutant monomelic streptavidin has a lower off rate for biotin or a biotin conjugate than monomelic streptavidin. The invention also encompasses nucleic acid molecules comprising a nucleotide sequence encoding the mutant streptavidin subunit and vectors and cells comprising the nucleic acid. Further, methods of capturing biotinylated molecules and cells and therapeutic methods are encompassed.