Providing improved immunoglobulin sequences by mutating CDR and/or FR positions

The present invention relates to methods and techniques for providing improvedamino acid sequences that can be used as single antigen-binding domains. In particular, the invention relates to methods and techniques for providing improved amino acid sequences that can be used as single antigen-binding domains that comprise or essentially consist of at least one immunoglobulin sequence. More in particular, the amino acid sequences provided herein may comprise or essentially consist of at least one variable domain sequence or a suitable fragment thereof, such as at least one light chain variable domain sequence (e.g. a V -sequence) or a suitable fragment thereof, or at least one heavy chain variable domain sequence (e.g. a V-sequence or V sequence)or a suitable fragment thereof.