Molecular recognition between nucleobases plays a crucial role in all kinds of biological processes.However,real-space investigation of the recognition capability of nucleobases in the presence of interfering compounds remains unexplored.Herein,based on the combination of scanning tunneling microscopy imaging and density functional theory modeling,we report the impact of the presence of melamine (M) on the formation and chirality of guanine (G)-tetrads on Au(111).Although M can interact with G by double hydrogen bonding,the Hoogsteen base pairing of G is not compromised,forming identical individual G-tetrads as would have happened without the presence of M.G-tetrads coexist with M on the surface not only in separate domains,but also within the mixture network of G-tetrads and M-dimers.Although the adsorption orientation of G-tetrads in the mixture network diversifies into two distinct angles,all G-tetrads in the network keep the same chirality,emphasizing the high preference of homochirality in such biochemical systems.