In another study, the inflammatory regulatory proteins α2-HS-glycoprotein (AHSG), α1 acid glycoprotein (AGP), apolipoprotein A-1 (APOA1), and haptoglobin (HP) were found to be differentially expressed in the serum of patients with NPDR and PDR compared to healthy controls. Here, APOA1 is linked to non-proliferative diabetic retinopathy.