Single-domain antigen-binding proteins that bind mammalian IgG

An antigen-binding protein comprising an amino acid sequence that comprises 4 framework regions, FR1 to FR4, and 3 complementarity determining regions, CDR1 to CDR3, that are operably linked in the order FR1-CDR1-FR2-CDR2-FR3-CDR3-FR4, wherein:

• (i) the CDR1 has an amino acid sequence as set forth in SEQ ID NO:
• 1;
• (ii) the CDR2 has an amino acid sequence as set forth in SEQ ID NO:
• 50;
• and,
• (iii) the CDR3 is an amino acid sequence as set forth in SEQ ID NO:
• 99;
• and,
• wherein:
• FR1 comprises an amino acid selected from the group consisting of:
• (a) an amino acid having SEQ ID:
• 197;
• (b) an amino acid having at least 90% sequence identity with SEQ ID:
• 197;
• and/or,
• (c) the amino acid sequence of a) that has one or more amino acid substitutions as defined in Table 1;
• FR2 comprises an amino acid selected from the group consisting of:
• (d) an amino acid having SEQ ID:
• 198;
• (e) an amino acid having at least 90% sequence identity with SEQ ID:
• 198;
• and/or,
• (f) the amino acid sequence of d) that has one or more amino acid substitutions as defined in Table 2;
• FR3 comprises an amino acid selected from the group consisting of the amino acid sequence:
• (g) an amino acid having SEQ ID:
• 199;
• (h) an amino acid having at least 90% sequence identity with SEQ ID:
• 199;
• and/or,
• (i) the amino acid sequence of g) that has one or more amino acid substitutions as defined in Table 3;
• and,
• FR4 comprises an amino acid selected from the group consisting of:
• (j) an amino acid having SEQ ID:
• 200;
• (k) an amino acid having at least 90% sequence identity with SEQ ID:
• 200;
• and/or,
• l) the amino acid sequence of j) that has one or more amino acid substitutions as defined in Table 4, and wherein the antigen-binding protein specifically binds to the Fc domain of a human IgG molecule.