Side effects from targeted drugs remain a serious concern.One reason is the nonselective binding of a drug to unintended proteins such as its paralogs,which are highly homologous in sequences and have similar structures and drug-binding pockets.To identify targetable differences between paralogs,we analyzed two types (type-Ⅰ and type-Ⅱ) of functional divergence between two paralogs in the known target protein receptor family G-protein coupled receptors (GPCRs) at the amino acid level.Paralogous protein receptors in glucagon-like subfamily,glucagon receptor (GCGR) and glucagon-like peptide-1 receptor (GLP-1R),exhibit divergence in ligands and are clinically validated drug targets for type 2 diabetes.Our data showed that type-Ⅱ amino acids were significantly enriched in the binding sites of antagonist MK-0893 to GCGR,which had a radical shift in physicochemical properties between GCGR and GLP-1R.We also examined the role of type-Ⅰ amino acids between GCGR and GLP-1R.The divergent features between GCGR and GLP-1R paralogs may be helpful in their discrimination,thus enabling the identification of binding sites to reduce undesirable side effects and increase the target specificity of drugs.