Binding molecule conjugates

A surrogate light chain construct comprising:

• a heavy chain variable domain;
• and
• a surrogate light chain (SLC) having at least one amino acid residue at a predetermined amino acid position for conjugation to an agent, wherein the amino acid residue is located
• (i) at a 16th and/or 21st position of an amino acid sequence of a VpreB1 from which a secretory leader is absent;
• and/or
• (ii) at one or more positions selected from the group consisting of a 60th, 74th, 78th, 79th, 85th, 91st, 110th, 123rd, 131st, 133rd, 166th, and 170th position of an amino acid sequence of a λ5 from which a secretory leader is absent,
• wherein the heavy chain variable domain and SLC comprise a binding region.