The present disclosure provides an asymmetric mixed antibody comprising two heavy chains or heavy chain fragments each comprising at least a variable region, a hinge region and a CH1 domain, wherein a first heavy chain or fragment thereof is a class IgG4 and has: a the inter-chain cysteine at position 127, numbered according to the Kabat numbering system, in the CH1 domain is substituted with another amino acid; and b optionally one or more of the amino acids positioned in the upper hinge region is substituted with cysteine, and wherein the second heavy chain or fragment thereof is characterised in that part or all of the chain has a different amino acid sequence to said first heavy chain in at least the region outside the variable region (for example the constant region), formulations comprising the same, therapeutic used of both of the above, and processes for preparing the antibodies and formulation.