Antigen binding molecules

A pair of binding molecules, comprising

• (a) a first binding molecule comprising (i) a first antigen binding domain capable of binding to a target antigen, (ii) a first part of an effector domain, and (iii) a first complementing domain capable of association with the first part of the effector domain;
• and
• (b) a second binding molecule comprising (i) a second antigen binding domain capable of binding to a target antigen, (ii) a second part of an effector domain, and (iii) a second complementing domain capable of association with the second part of the effector domain;
• wherein the first and the second part of the effector domain are capable of associating with each other to form a functional effector domain if the first and the second antigen binding domain bind to their target antigens on the surface of a cell,
• wherein the first and the second complementing domain are associated with the first and the second part of the effector domain, respectively, while the first and the second part of the effector domain are not associated with each other;
• and
• wherein the first and the second complementing domain are not capable of forming a functional effector domain with a part of the effector domain.