Angiosarcoma (AS) is a rare, genomically complex and aggressive vascular sarcoma in humans. The limited availability of patients impaired the development of effective therapies and, consequently, the survival rate is dramatically poor. Dogs spontaneously develop a histopathologically similar neoplasia, hemangiosarcoma (HSA), that is relatively common, representing 5-7% of all canine malignant tumors, with an incidence that is even significantly higher in breeds such as German shepherd and boxer. Both AS and HSA are considered to originate from hematopoietic endothelial progenitor cells. Despite the fact that AS is predominantly cutaneous in humans and HSA visceral in dogs, metastasis are mostly localized in lungs and liver in both models and comparative genomic analyses confirmed strong similarities between AS and HSA as well as the high but convergent heterogeneity within and between patients of both species. The dog as a spontaneous large animal cancer model has a lot of advantages over the more commonly used rodent models in which tumors are artificially induced. The project's general aim is to isolate a set of immunoreagents suitable for the characterization, diagnostics and, potentially, therapy of HSA in dogs. They should target biomarkers exposed at the cell surface, allowing the selective targeting of the corresponding cells in tissues. Furthermore, they could serve as reagents for implementing similar diagnostic protocols in human AS.