Despite 6 products on the market and dozens in advanced phases of clinical development, in vivo gene therapy with vectors derived from adeno-associated viruses (AAV) still faces the major challenge of the limited vector cargo capacity (< 5kb). This prevents AAV application for treatment of inherited diseases caused by mutations in large genes. To address this, we have recently developed an AAV-based strategy relying on the use of short protein elements called inteins. These, when fused at the extremities of fragments of a large protein delivered to cells through AAV, can mediate joining of the fragments in a traceless manner, resulting in the reconstitution of the intact target protein. However, despite being extensively utilized as biotechnological tools in vitro, the in vivo application of inteins for therapeutic purposes comes with several limitations. These include: insufficient levels of full-length protein reconstitution for some applications; complex design; and production of undesired, potentially harmful by-products. NextGeneTx aims to expand the potential of AAV-inteins as an innovative in vivo biotechnological tool for therapeutic purposes, by addressing each of these limitations. To achieve this, we will leverage cutting-edge technologies to: i. engineer inteins with improved efficiency and greater design flexibility; ii. modulate intracellular trafficking of intein-containing protein fragments to enhance the efficiency and accuracy of full-length protein reconstitution; iii. develop strategies to reduce the production of unwanted by-products. We will test the therapeutic relevance of evolved AAV-inteins in both gene therapy and genome editing applications, using human retinal organoids and animal models of retinal diseases. The outcomes of NextGeneTx will define a highly adaptable, safe, and effective next-generation AAV-based approach to deliver large genes, thus significantly expanding the patient population that can benefit from in vivo gene therapy.