C9 and Fabry disease: <h4>Background</h4>Enzyme replacement therapy (ERT) for Fabry disease can elicit anti-drug antibodies (ADAs) that may diminish efficacy and limit clinical benefit.<h4>Objectives</h4>To develop and apply a multiplexed proteomic assay for quantitative, subclass-specific ADA and complement profiling in Fabry disease to inform personalized ERT selection.<h4>Methods</h4>We created a targeted LC-MS/MS platform to quantify ADA binding across IgG1-4, IgM, and IgA1, and complement proteins C1Qc-C9, in serum from Fabry patients (n = 39) and healthy controls.