Immunoglobulin binding protein

The present invention is directed to an immunoglobulin light chain binding protein which comprises the amino acid sequence of SEQ ID NO:1 modified by an amino acid substitution at one or more of positions 39, 53 and 57 and/or by an amino acid insertion between positions 59 and 60 such that the dissociation constant (Kd) of the protein with respect to human immunoglobulin -chain is 400 nM or more at pH 8, or the amino acid sequence of a corresponding immunoglobulin light chain binding domain modified by an amino acid substitution at one or more of the positions equivalent to positions 39, 53 and 57 of SEQ ID NO:1 and/or by an amino acid insertion between positions equivalent to positions 59 and 60 of SEQ ID NO:1, such that the dissociation constant (Kd) of the protein with respect to human immunoglobulin -chain is 400 nM or more at pH 8, or the amino acid sequence of a fragment of (a) or (b) which contains at least one said substitution and/or insertion, such that the dissociation constant (Kd) of the protein with respect to human immunoglobulin -chain is 400 nM or more at pH 8.