The epidermal growth factor receptor EGFR is a receptor tyrosine kinase found on the cell surface of vertebrates. Binding of a ligand, like EGF, prompts activation of a variety of signalling pathways, ultimately leading to cellular growth and survival. When the receptor is overexpressed or mutated, aberrant activation of cellular signalling pathways can lead to cancerous cell growth. Currently, anti-EGFR antibodies like cetuximab as well as tyrosine kinase inhibitors (TKIs) are used in order to halt tumor progression. Though, the emergence of resistance presents a major challenge concerning the use of EGFR inhibitors. Therefore, the development of non-cross-resistant combination therapy is crucial in order to stop or at least delay the onset of resistance. Only few models exist, which allow the prediction of resistance mutations. Therefore, this thesis aimed at the development of an in-vitro assay, which allows the prognosis of resistance mutations emerging upon EGFR-targeted therapy in cancer. An EGFR library, which had been randomly mutated using error-prone PCR, was used for the transfection of HEK293T cells. In order to express only one EGFR mutant per cell, the transfection procedure was optimized to yield only one plasmid per cell. The transfected cell population, expressing mutant EGFR on the cell surface, was subjected to a selection pressure (i.e. a TKI) and the resulting intracellular phosphorylation signal exhibited by EGFR was detected via flow cytometry. Access to the cell interior was granted through prior fixation and permeabilization. Only EGFR mutants harbouring resistance mutations showed a phosphorylation signal in the presence of an EGFR inhibitor and hence were selected and used for the isolation of functional DNA. After selection, the intact genomic material will be amplified using PCR and further subjected to a subsequent selection cycle. Ultimately, the DNA shall be analysed and the mutations conferring resistance can be determined.