PROS1 and systemic lupus erythematosus: As the TAM kinases may be important in the disposition of apoptotic cells, we evaluated plasma concentrations of their ligands Gas6 and free protein S. Although the levels of Gas6 and free protein S were not different overall between patients with SLE and matched healthy controls, free protein S was decreased in subsets of SLE patients with a history of serositis, neurologic, hematologic, and immunologic disorder.