knobs-into-holes 

The knobs-into-holes technique is suitable for heterodimerization immunoglobulin heavy (H) chains with amino acid (AA) changes, which can overcome the mispairing problem. In order to overcome the impurity problem in tri-functional antibody production process, experts from Creative Biolabs introduce a knob into the CH3 of one antibody and a corresponding hole into the CH3 of the other antibody via the alteration of AA respectively. We then co-transfect these constructs encoding all four chains either into E.coli or mammalian cells such as human embryonic kidney (HEK) cells. We can also provide glycosylated, functional and competent heterodimeric antibodies after the expression step in mammalian host.
https://www.creativebiolabs.net/knobs-in-holes-bispecific-antibody-kih-bsab.htm