This facilitates the development of resistance, which then leads to clinical relapse and progression of the disease. Therefore, AnxA2 could potentially be used as a diagnostic marker as well as therapeutic target in Her-2 negative cancers. To reveal how proteins interact in a protein complex, the detailed structure of the Butenafine hydrochloride complex is ideally determined via crystallography methods or NMR. However, the structure determination of protein complexes remains challenging and the number of complex structures lags far behind the number of known protein interactions. This gap will grow as interactomics projects lead to a vast increase in the number of known protein-protein interactions. Alternative methods are developed for prediction of protein complex structures to -at least partiallybridge this gap. In silico methods such as homology based modeling and protein-protein Mechlorethamine hydrochloride docking can predict the structure of protein complexes. Additionally, fitting of monomer structures or models into low resolution structures of the complex obtained via SAXS, cryo-electron microscopy or electron tomography can provide a model for the complex. Models from these predictions can further be validated by experimental methods, such as mutagenesis of the predicted interface combined with a method to detect the specific protein-protein interaction. Conversely, experimental identification of interface residues can help to guide the docking process in data-driven docking, often resulting in better models. The development of new methods to determine interfaces in protein-protein interactions can thus contribute to the development of alternative methods for complex structure modeling. We here propose a new random mutagenesis strategy to identify putative interface residues based on the mammalian two-hybrid method MAPPIT. MAPPIT is a two-hybrid method based on reconstitution of cytokine receptor signaling for the detection of protein-protein interactions. The MAPPIT principle is outlined in figure S1 in supporting information. We previously used MAPPIT and site directed mutagenesis to identify an interface in the human host restriction factor Apobec3G that is important for its dimerization and its interaction with the HIV-1 protein Vif. Human apolipoprotein B messenger RNA-editing catalytic polypeptidelike G is a member of the Apobec protein family of cytidine deaminases. Apobec3G is a host restriction factor that inhibits the infectivity of HIV-1 virus particles that lack the accessory protein virion infectivity factor. Apobec3G is incorporated into newly formed HIV-1 virions and catalyzes cytidine deamination during reverse transcription of the viral genome in infected cells. This leads to hypermutation and degradation of the newly synthesized viral DNA. Apobec3G further restricts HIV-1 infection through deaminaseindependent mechanisms. Unfortunately, HIV-1 can efficiently counteract the restrictive effects of Apobec3G by Vif. HIV-1 Vif is a 23 kDa protein that targets Apobec3G for proteasomal degradation. Vif binds to Apobec3G and recruits via its SOCS box domain an E3 ubiquitin ligase complex with Cullin-5, Elongin B, Elongin C and Rbx1 subunits. This leads to the ubiquitination of Apobec3G and degradation by the 26S proteasome. Apobec3G contains two characteristic cytidine deaminase domains. Only the C-terminal CDA domain is catalytically active in cytidine deamination, whereas the Nterminal CDA domain is involved in nucleic acid binding and virion incorporation. Virion incorporation of Apobec3G is mediated via the RNA-dependent interaction with the conserved nucleocapsid domain of the HIV-1 Gag protein. The nucleocapsid domain is necessary and sufficient for interaction with and incorporation of Apobec3G in virus-like particles.