Patient description: The patient is a 35-year-old woman with myasthenia gravis, class IIa. She complains of diplopia and fatigue and weakness that affects mainly her upper limbs. She had a positive anti-AChR antibody test, and her single fiber electromyography (SFEMG) was positive. She takes pyridostigmine 60 mg three times a day. But she still has some symptoms that interfere with her job. She is a research coordinator and has 3 children. Her 70-year-old father has hypertension. She does not smoke or use illicit drugs. She drinks alcohol occasionally at social events. Her physical exam and lab studies were 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): 3.0-35.0,  Neurofibroma, Plexiform Precursor Cell Lymphoblastic Leukemia-Lymphoma Leukemia, Prolymphocytic, Acute Sarcoma Diagnosis Phase I: Patients must have recurrent or refractory solid tumors or acute leukemia (limited to AML or ALL) or have been intolerant of prior therapies, confirmed by the Laboratory of Pathology, NCI, e.g., solid tumors including rhabdomyosarcoma, Ewing sarcoma, soft tissue sarcomas. These may primary neoplasms of the central nervous system, such as high-grade (WHO grade III-IV) glioma. Patients with diffuse intrinsic pontine glioma (DIPG) or optic pathway glioma are exempt from histologic verification. For DIPG typical MRI findings must be present which hypo or isointense on T1-weighted imaging, hyperintense on FLAIR or T2-weighted imaging, epicenter in the pons in the face of a typical clinical presentation. Optic pathway glioma are located in the optic pathway and are typically hypo or iso-intense on T1 and hyperintense on T2-weighted images In addition, patients with NF1 and with malignant peripheral nerve sheath tumor (MPNST) Phase II: inoperable PN causing morbidity, such as (but not limited to) head and neck lesions that could compromise the airway or great vessels, brachial or lumbar plexus lesions that could cause nerve compression and loss of function, lesions that could result in major deformity (e.g., orbital lesions) or significant cosmetic problems, lesions of the extremity that cause limb hypertrophy or loss of function, and painful lesions in patients with NF1 Histologic confirmation of PN tumor is not necessary in the presence of consistent clinical and radiographic findings but should be considered if malignant degeneration of a PN is clinically suspected A PN is defined as a neurofibroma that has grown along the length of a nerve and may involve multiple fascicles and branches. A spinal PN involves two or more levels with connection between the levels or extending laterally along the nerve. In addition to PN, all study subjects must have either positive genetic testing for NF1 confirmed in a CLIA certified laboratory or have at least one other diagnostic criterion for NF1 listed below (NIH Consensus conference) Six or more cafe-au-lait macules (greater than or equal to 0.5cm in prepubertal subjects or greater than or equal to 1.5 cm in post pubertal subjects) Freckling in axilla or groin  Individuals who are pregnant or breast feeding or who become pregnant while enrolled on this trial will be excluded from participation, due to the unknown effects of PLX3397 on a growing fetus or newborn child Individuals with malignant peripheral nerve sheath tumors will not be eligible to participate in the phase II portion of the trial Ongoing treatment with any other cancer therapy or investigational agent, with the exception of IT chemotherapy for leukemia, when indicated Individuals who require therapy with warfarin Uncontrolled intercurrent illness including, but not limited to, ongoing or active infection, symptomatic congestive heart failure, unstable angina pectoris, cardiac arrhythmia, or psychiatric illness/social situations that would limit compliance with study requirements Active untreated infection Known chronic Hepatitis B or C, or HIV infection History of allergic reactions attributed to compounds of similar chemical or biologic composition to PLX3397 or other agents used in study Patients with PT and/or INR higher than or equal to 1.5 time upper limit of normal, unless patients have lupus anticoagulant in which case they are eligible if cleared by hematology Drugs that strongly inhibit or potentiate CYP3A4