To map the human MuSK mAb epitopes, MuSK constructs that had particular domains deleted and full-length MuSK were each expressed in HEK cells and tested with the CBA. (A) The schematic illustrates the mutant forms of MuSK. For example, “ΔIg-1” includes only the Ig-like domains 2 and 3 and the frizzled-like (Fz-like) domain because the Ig-like domain 1 was deleted (shown as “Δ” in the schematic). Similarly, “Ig-1” includes only the Ig-like domain 1 because the Ig-like domains 2 and 3 and Fz-like domain were deleted (shown as “Δ” in the schematic). Binding of mAbs (MuSK1A, MuSK1B, MuSK3-28, and the positive control humanized MuSK mAb 4A3) to these mutant forms of MuSK was tested in our standardized flow cytometry CBA. Results for each (B) mAb or (C) serum specimen are shown. Serum was obtained from the same patients from whom the mAbs were derived. Each data point represents a separate replicate within the same experiment. Bars represent means and error bars SDs.