 To avoid this, parts of the antibody can be replaced with human amino acid sequences, or pure human antibodies can be engineered.If the constant region is replaced with the human form, the antibody is termed "chimeric" and the substem used was "-xi-".Part of the variable regions may also be substituted, in which case it is called "humanized" and "-zu-" was used; typically, everything is replaced except the complementarity-determining regions (CDRs), the three loops of amino acid sequences at the outside of each variable region that bind to the target structure, although some other residues may have to remain non-human in order to achieve good binding.