Abstract Approximately 13,000 people this year in the United States alone will be diagnosed with sarcoma and nearly 5,000 will lose their lives to this devastating disease. Soft tissue sarcomas are a clinically and biologically heterogeneous group of tumors made up of approximately fifty subtypes that are classified by their presumed mesenchymal origin. The development of novel therapies for sarcoma has been hampered by the considerable heterogeneity of the disease and the limited understanding of the molecular mechanisms driving sarcoma formation and evolution. Due to this, clinically, the majority of patients with aggressive soft tissue sarcoma subtypes are treated similarly despite evidence these subtypes are unique. Our lab has focused on the development of a new model for the study of sarcoma subtypes using a primary human transformation system. Based on TCGA data, one characteristic that is shared between sarcoma subtypes is the loss or silencing of p53 and Rb1. We have started with immortalized mesenchymal stem cells (MSC), the presumed cell of origin for sarcoma and introduced lentiviral constructs targeting p53 and Rb1 by CRISPR technology to delete these genes. Following this, we have added a barcoded oncogene library with genes implicated in sarcoma specific biology or across cancer subtypes. Using this system, we have successfully transformed these MSCs into high grade soft tissue sarcomas. The current goal is to continue to manipulate this system to develop multiple soft tissue sarcoma subtypes. By starting with the same cell of origin and arriving at different sarcoma subtypes, we can begin to understand the relationship between these subtypes which will inform clinical trial design. Using this system, we can investigate the development of sarcoma, understand subtype heterogeneity, and utilize this tool to develop novel therapies for our patients. Citation Format: Janai Carr-Ascher. Development of a phenotypically diverse model of sarcoma formation to identify key drivers of tumorigenesis across subtypes [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2714.