Patient description: The patient is a 16-year-old girl recently diagnosed with myasthenia gravis, class IIa. She complains of diplopia and weakness affecting in her upper extremities. She had a positive anti-AChR antibody test, and her single fiber electromyography (SFEMG) was positive. She is on acetylcholinesterase inhibitor treatment combined with immunosuppressants. But she still has some symptoms. She does not smoke or use illicit drugs. She is not sexually active, and her menses are regular. Her physical exam and lab studies are not remarkable for any other abnormalities. BP: 110/75 Hgb: 11 g/dl WBC: 8000 /mm3 Plt: 300000 /ml Creatinine: 0.5 mg/dl BUN: 10 mg/dl Beta hcg: negative for pregnancy

Clinical trial eligibility criteria: eligible ages (years): 1.0-21.0,  Advanced Malignant Solid Neoplasm Ann Arbor Stage III Childhood Non-Hodgkin Lymphoma Ann Arbor Stage IV Childhood Non-Hodgkin Lymphoma Low Grade Glioma Malignant Glioma Recurrent Childhood Ependymoma Recurrent Childhood Malignant Germ Cell Tumor Recurrent Childhood Medulloblastoma Recurrent Childhood Non-Hodgkin Lymphoma Recurrent Childhood Rhabdomyosarcoma Recurrent Childhood Soft Tissue Sarcoma Recurrent Ewing Sarcoma/Peripheral Primitive Neuroectodermal Tumor Recurrent Hepatoblastoma Recurrent Langerhans Cell Histiocytosis Recurrent Malignant Solid Neoplasm Recurrent Neuroblastoma Recurrent Osteosarcoma Recurrent Primary Central Nervous System Neoplasm Refractory Langerhans Cell Histiocytosis Refractory Malignant Solid Neoplasm Refractory Neuroblastoma Refractory Non-Hodgkin Lymphoma Refractory Primary Central Nervous System Neoplasm Rhabdoid Tumor Stage III Soft Tissue Sarcoma AJCC v7 Stage IV Soft Tissue Sarcoma AJCC v7 Wilms Tumor Patient must have enrolled onto APEC1621SC and must have been given a treatment assignment to molecular analysis for therapy choice (MATCH) to APEC1621B based on the presence of an actionable mutation Patients must have a body surface area >= 0.53 m^2 at enrollment Patients must have radiographically measurable disease at the time of study enrollment; patients with neuroblastoma who do not have measurable disease but have metaiodobenzylguanidine (MIBG) positive (+) evaluable disease are eligible; measurable disease in patients with CNS involvement is defined as tumor that is measurable in two perpendicular diameters on magnetic resonance imaging (MRI) and visible on more than one slice Note: The following do not qualify as measurable disease Malignant fluid collections (e.g., ascites, pleural effusions) Bone marrow infiltration except that detected by MIBG scan for neuroblastoma Lesions only detected by nuclear medicine studies (e.g., bone, gallium or positron emission tomography [PET] scans) except as noted for neuroblastoma Elevated tumor markers in plasma or cerebrospinal fluid (CSF) Previously radiated lesions that have not demonstrated clear progression post radiation Leptomeningeal lesions that do not meet the measurement requirements for Response Evaluation in Solid Tumors (RECIST) 1.1  Pregnant or breast-feeding women will not be entered on this study due to risks of fetal and teratogenic adverse events as seen in animal studies; pregnancy tests must be obtained in girls who are post-menarchal; males or females of reproductive potential may not participate unless they have agreed to use an effective contraceptive method, while receiving study treatment and for 3 months after the last dose of JNJ-42756493 (erdafitinib); male subjects (with a partner of child-bearing potential) must use a condom with spermicide when sexually active and must not donate sperm from the first dose of study drug until 5 months after the last dose of study drug Concomitant medications Corticosteroids: patients receiving corticosteroids who have not been on a stable or decreasing dose of corticosteroid for at least 7 days prior to enrollment are not eligible; if used to modify immune adverse events related to prior therapy, >= 14 days must have elapsed since last dose of corticosteroid Investigational drugs: patients who are currently receiving another investigational drug are not eligible Anti-cancer agents: patients who are currently receiving other anti-cancer agents are not eligible Anti-GVHD agents post-transplant: patients who are receiving cyclosporine, tacrolimus or other agents to prevent graft-versus-host disease post bone marrow transplant are not eligible for this trial CYP3A4 agents: patients who are currently receiving drugs that are strong inducers or inhibitors of CYP3A4 are not eligible; Note: CYP3A4 inducing anti-epileptic drugs and dexamethasone for CNS tumors or metastases, on a stable dose, are allowed CYP2C9 agents: patients who are currently receiving drugs that are moderate to strong inducers or inhibitor of CYP2C9 are not eligible P-glycoprotein: patients who are currently receiving drugs that are potent inhibitors of p-glycoprotein are not eligible Patients who have an uncontrolled infection are not eligible