PROS1 and HIV infectious disease: This is similar to the findings of Comp et al., who noted elevated levels of free protein S in familial C4BP deficiency.23 Similarly, Ceriello et al. demonstrated that low free protein S levels and activity were associated with elevated C4BP concentrations in type 1 diabetes mellitus.24 This finding suggests that the protein S deficiency found in HIV infection may contribute to elevated C4BP.