The present invention provides an antibody of the class IgG4 comprising at least one heavy chain which comprises a CH1 domain and a hinge region, wherein in each heavy chain: 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. one or more of the amino acids positioned in the upper hinge region is substituted with cysteine.
