PROS1 and melanoma: LC growth factors were not expressed at high levels by in vivo tumors; however, we observed significant increases in Pros1 and Gas6; both TAM receptor ligands have previously been associated with the suppression of anti-tumor immunity (73, 74), and Pros1 is known to inhibit differentiation of bone marrow–derived LCs (75), suggesting that growing melanomas may directly impair LC function in situ before migration.