PROS1 and infection: The power of these RFs is two-fold: (1) they are constitutively expressed in the immune cells targeted by HIV/SIV, thereby allowing them to function at the moment of infection as intrinsic defense proteins, and (2) they are induced by type I interferons, a function that renders them members of the interferon-stimulated gene family that are essential components of the innate immune system [12,17,22,58].