Gene therapy is a promising treatment option for cancer. Ideally, a therapeutic gene is delivered specifically into tumor cells sparing the neighboring normal cells. For this purpose gene delivery vectors are designed that can recognize structures, which are exclusively expressed on tumor cells (i.e. the tumor-associated antigens -TAA-). Retroviral vectors are commonly used for gene therapy by modifying the envelope protein responsible for the recognition of the target cell. The Vesicular Stomatitis Virus G protein (VSV-G) is a well-liked choice for pseudotyping the retroviral vectors since it confers on the viral particle stability to allow concentration to high titers necessary for the clinical applications. However, the main drawback of VSV-G, the ubiquitously expressed receptor and thus the broad target range, hinders the use of this protein for targeted gene therapy. In this thesis, we aimed to modify the VSV-G for targeted gene therapy against Prostate Stem Cell Antigen (PSCA) -expressing tumors. Therefore we followed two approaches. The first approach comprised of the fusion of a single-chain antibody fragment against PSCA to the N-terminus of VSV-G. In the second approach the VSV-G was modified by insertion of a small epitope. We could demonstrate that two positions in the N-terminal region of VSV-G protein permit insertion of a ten amino acid long epitope. These mutant VSV-G proteins were successfully assembled into retroviral particles. We demonstrated that the mutant retroviral particles can be used for targeting to PSCA-positive cells using nanobeads. The nanobeads were chemically coupled to antibodies against the epitope in the VSV-G protein and PSCA on the tumor cell. These bispecific nanobeads allowed the recruitment of mutant retroviral particles to the PSCApositive cells. Our results point out the potential of these mutant retroviral particles in targeted gene delivery. Further studies will be necessary to assess the efficiency of in vivo targeted gene therapy using these mutant retroviral particles.