GAS6 and neoplasm: To support activation of TAM-dependent oncogenic pathways by PROS1 and GAS6 ligands, PtdSer may be provided by several sources: intra-tumoral apoptotic cells, tumor-associated endothelial cells which were found to be enriched for externalized PtdSer [40], tumor-derived exosomes which are densely coated with exposed PtdSer, or PtdSer exposed by viable tumor cells.