The present application provides specific sites for modifying antibodies or antibody fragments by replacing at least one native amino acid in the constant region of a parental antibody or antibody fragment with cysteine, which can be used as a site of attachment for a payload or linker-payload combination. In one embodiment, the antibodies are modified with cysteines at positions 152 and 375 of the heavy chain constant region, as defined by EU numbering format. In another embodiment, the antibodies are modified with cysteines at position 360 of the heavy chain constant region, and position 107 of the kappa light chain constant region, as defined by EU numbering format.