Colorectal cancer (CRC) is the third most frequently diagnosed cancer worldwide with rising incidence and the primary cause of cancer-related deaths in Germany. Disease relapse after an apparently successful treatment remains a major stumbling block for patients diagnosed in advanced stages. Resistance to therapy likely occurs owing to a small population of cancer cells with stem cell properties (CSCs), appearing as the relevant cell target for therapy. In vivo models showed that once CSC are eliminated, however, differentiated cells are able to dedifferentiate into a CSC state and drive tumor re-growth. To date, the molecular mechanisms underlying cell plasticity remain largely enigmatic. Ex vivo tumor organoid models suggest a role of a cell-autonomous component, significantly boosted by extracellular cues, like growth factors. These cues are transduced into cancer cells through surface receptors, which are highly dependent on co-receptors such as CD44 for their efficient function. The latter is also used as a marker of CSCs and represents a potential target through which cell plasticity could be modulated. In this research project, we will employ state-of-the-art adeno-associated virus (AAV) vector technology and tumor organoids to target specific genes with a pivotal role in CRC cell plasticity. A novel type of combinatorial AAV vectors will be genetically engineered to deliver, alone or in combination, the components of powerful techniques for gene regulation, namely, CRISPR and RNA interference (RNAi), providing an option to create new classes of gene therapeutics to target the plastic process. Since CD44 regulates several signaling pathways, manipulating CD44 will have an impact on these pathways and will help understanding their contribution along the process.