FASLG and autoimmune polyendocrinopathy: Another study investigated antigen specificity of aPL in COVID-19 and found that, contrary to APS, which is characterized by high aPL titers with specificity against domain 1 on β2-GPI, patients with COVID-19 exhibit low titers of anti-β2-GPI, with specificity against domains 4 and 5 (20).