TAT-D1 peptide is a selective D1-D2 heteromer antagonist. It disrupts D1-D2 receptor heterodimers by interacting with specific domains of the D2 receptor through its unique amino acid sequence, especially the two glutamic acid residues at the C-terminus of the D1 receptor. TAT-D1 peptide is used for studying the role of D1-D2 receptor heterodimers in regulating depression and anxiety-like behaviors.