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): 2.0-18.0,  Recurrent Malignant Solid Neoplasm Recurrent Melanoma Recurrent Primary Central Nervous System Neoplasm Recurrent Thyroid Gland Carcinoma Refractory Malignant Solid Neoplasm Refractory Primary Central Nervous System Neoplasm Thyroid Gland Medullary Carcinoma Patients must have a body surface area >= 0.44 m^2 when enrolling on dose level -1; patients must have a body surface area >= 0.35 m^2 when enrolling on dose level 1, 2, or 3 PART A: Patients with relapsed or refractory solid tumors (excluding medullary thyroid cancer) including CNS tumors and malignant melanoma are eligible; patients must have had histologic verification of malignancy at original diagnosis or relapse except in patients with intrinsic brain stem tumors, optic pathway gliomas, or patients with pineal tumors and elevations of cerebrospinal fluid (CSF) or serum tumor markers including alpha-fetoprotein or beta-human chorionic gonadotropin (HCG) Part B: Patients with medullary thyroid cancer (MTC), with or without bone marrow involvement, will be eligible for Part B; these patients will be enrolled at dose level 2, the recommended phase 2 dose determined in the dose escalation part of the study Patients must have either measurable or evaluable disease Patient's current disease state must be one for which there is no known curative therapy or therapy proven to prolong survival with an acceptable quality of life Karnofsky >= 50% for patients > 16 years of age and Lansky >= 50 for patients =< 16 years of age Note: neurologic deficits in patients with CNS tumors must have been relatively stable for at least 7 days prior to study enrollment; patients who are unable to walk because of paralysis, but who are up in a wheelchair, will be considered ambulatory for the purpose of assessing the performance score Patients must have fully recovered from the acute toxic effects of all prior anti-cancer chemotherapy Myelosuppressive chemotherapy: at least 21 days after the last dose of myelosuppressive chemotherapy (42 days if prior nitrosourea) Hematopoietic growth factors: at least 14 days after the last dose of a long-acting growth factor (e.g. Neulasta) or 7 days for short-acting growth factor; for agents that have known adverse events occurring beyond 7 days after administration, this period must be extended beyond the time during which adverse events are known to occur; the duration of this interval must be discussed with the study chair  Pregnant or breast-feeding women will not be entered on this study; 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 two methods of birth control a medically accepted barrier method of contraceptive method (e.g., male or female condom) and a second effective contraceptive method of birth control during protocol therapy and for at least 4 months after the last dose of XL184; abstinence is an acceptable method of birth control 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 Patients who are currently receiving another investigational drug are not eligible Patients who are currently receiving other anti-cancer agents are not eligible 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 Patients must not be receiving any of the following potent cytochrome P450 family 3, subfamily A, polypeptide 4 cytochrome (CYP3A4) inducers or inhibitors: erythromycin, clarithromycin, ketoconazole, azithromycin, itraconazole, grapefruit juice or St. John's wort Patients who are receiving systemic treatment anticoagulation are not eligible; patients receiving prophylactic systemic anticoagulation will be allowed as long as PT/INR requirements are met Patients must not have received enzyme-inducing anticonvulsants within 14 days prior to enrollment