SOMAscan Assay:

The SOMAscan proteomic assay (SomaLogic, Boulder, CO) is designed to measure concentrations of different amount of proteins in a small volume of serum sample [12]. In brief, each of the 1,125 proteins measured in serum by the version of the SOMAscan assay performed in this study has its own targeted SOMAmer reagent, which is used as an affinity binding reagent and quantified on a custom Agilent hybridization chip. SOMAmer reagents include modified nucleotides which form complexes with the proteins in solution, and the concentration of the hybridized protein and reagent are quantified by microarray or quantitative polymerase chain reaction (qPCR) [12]. DMD and control serum samples were prepared at collection sites. In the SOMAscan assay, samples were randomly assigned to plates within each assay and run along with a set of calibration and normalization samples. No identifying information was available to the laboratory technicians operating the assay. Intra-run normalization and inter-run calibrations were performed according to SOMAscan v3 assay data quality control procedures as defined in the SomaLogic good laboratory practice quality system.