Cell‐based assay results. IgG antibody binding (red) of the patient’s serum (bottom) and healthy control serum (top) to human embryonic kidney 293 (HEK293) cells expressing MuSK‐EGFP (green). For this study, HEK293 cells were transfected with complementary DNAs (cDNAs) expressing human acetylcholine receptor α, β, δ, and ε/γ subunits, and rapsyn‐EGFP in a ratio 2:1:1:1:1; full length MuSK‐EGFP [5], and cDNAs expressing human lipoprotein receptor‐related protein 4 with a chaperone protein (low‐density lipoprotein receptor–related protein associated protein 1) to enhance cell surface expression. Serum was diluted 1:20 and scored 3 based on a score system from 0 = negative to 4 = strong labeling of almost all transfected cells as previously described [4]. EGFP, enhanced green fluorescent protein; IgG, immunoglobulin G; MuSK, muscle‐specific tyrosine kinase.