Muscle-specific tyrosine kinase (MuSK) myasthenia gravis (MG) is a rare but potentially severe disease, in which patients develop pathogenic autoantibodies that specifically target the MuSK protein in the neuromuscular junction. This phase 1 study is being conducted to evaluate the safety of various dosing regimens of an investigational cell therapy, MuSK-CAART, that can be given to patients with anti-MuSK antibody positive Myasthenia Gravis (MuSK MG), who have active disease. Various dosing regimens of MuSK-CAART alone, in combination with cyclophosphamide (CY), and in combination with CY and fludarabine (FLU) will be evaluated. Treatment with MuSK-CAART may potentially lead to complete and durable remission of disease.