We studied a passive transfer mouse model of MuSK MG, induced by injecting wildtype C57BL/6 mice with two different recombinant, monovalent pathogenic antibodies to MuSK, derived from a MuSK MG patient (17, 24, –26).These recombinant antibodies, like most antibodies in sera from MuSK MG patients, are directed against the first Ig-like domain in MuSK and inhibit Agrin-stimulated MuSK phosphorylation (17, 24, 27). This evidence concerns the gene AGRN and myasthenia gravis.