A polynucleotide encodes a modified antibody, or single chains thereof. The modified antibody has a non-antigen-binding peptide such as the constant regions of an antibody of a first species, peptide hormones, enzymes, and peptide transmitters; and a binding peptide such as the unsubstituted light and heavy chains of the variable region of an antibody of a second species which binds the human milk fat globule (HMFG) antigen. The non-antigen-binding peptide is linked to at least one chain of the binding peptide, the chains may be linked to one another at a site other than the antigenic binding site, and at least one chain of the binding peptide has 1 to 46 amino acids substituted with amino acids selected from specific ones assigned to each site. The polynucleotide and other products are also provided in the form of compositions, with a carrier. The polynucleotides may be RNAs and DNAs, and are also provided as hybrid vectors carrying them, and as transfected cells expressing the modified antibodies or their single chains.