RecruitingPhase 1Drug trialPhase I Trial of TURALIO(R) (Pexidartinib, PLX3397) in Children and Young Adults With Refractory Leukemias and Refractory Solid Tumors Including Neurofibromatosis Type 1 (NF1) Associated Plexiform Neurofibromas (PN) and Tenosynovial Giant Cell Tumor ...Background: \- Some people with cancer have solid tumors. Others have refractory leukemia. This may not go away after treatment. Researchers want to see if a drug called TURALIO(R) can shrink tumors or stop them from growing. Objectives: \- To find the highest safe dose and side effects of TURALIO(R). To see if it helps treat certain types of cancer. Eligibility: \- People ages 3-35 with a solid tumor or leukemia that has returned or not responded to cancer therapies. Design: * Individuals will be screened with: * Medical history * Physical exam * Blood and urine tests * Heart tests * Scans or other tests of the tumor * Individuals will take TURALIO(R) as a capsule once daily for a 28-day cycle. They can do this for up to 2 years. * During the study, participants will have many tests and procedures. They include repeats of the screening tests. Individuals will keep a diary of symptoms. * Individuals with solid tumors will have scans or x-rays. * Individuals with leukemia will have blood tests. They may have a bone marrow sample taken. * Some individuals may have a biopsy. * When finished taking TURALIO(R), individuals will have follow-up visits. They will repeat the screening tests and note side effects.