Circulating Pros1 is mainly secreted by hepatocytes where it acts as an anticoagulant, but Pros1 produced locally by other cells such as endothelial cells, osteoblasts, dendritic cells, T cells, vascular smooth muscle cells and tumour cells has no anticoagulant activity but instead functions as a cognate ligand for the TAM receptors Tyro3, Axl and Mertk. This evidence concerns the gene MERTK and neoplasm.