Patient description: The patient is a 14-year-old boy complaining of scoliosis and back pain. He has no other medical condition. He used to be able to play routine activities such as basketball and soccer, however, he recently has problem doing them. The pain is in his leg and back and aggravated by physical activities. He prefer lying down most of the time. He is not happy with his body gesture and complaints of shoulder imbalance and shifting his head to right. Patient is a well-dressed and well-nourished adolescent who is alert and cooperative. The left shoulder is slightly higher than the right shoulder. The scoliotic curve is measured as 45 degree. The patients is candidate for scoliosis surgery according to perioperative MEP monitoring

Clinical trial eligibility criteria: eligible ages (years): 0.0-999.0,  Langerhan's Cell Histiocytosis Juvenile Xanthogranuloma Erdheim-Chester Disease Rosai Dorfman Disease Neuro-Degenerative Disease Histiocytic Sarcoma Histiocytic Disorders, Malignant For Group 1: Participant must be less than 30 years of age at the time of enrollment For Group 2: Participant may be any age at the time of enrollment For Group 3: Participant must be less than 30 years of age at the time of enrollment For Group 4: Participant must be 30 years of age or older at the time of enrollment Participant must be able to take an enteral dose and formulation of medication. Study medication is only available as an oral suspension or tablet which may be taken by mouth or other enteral route such as nasogastric or gastric tube Biopsy proven LCH -AND Failure of at least front-line therapy for high-risk LCH with evaluable disease. -OR Failure of at least second-line therapy for low-risk LCH with evaluable disease. -OR Diagnosis of LCH-associated neurodegenerative disease with radiologic or clinical progression within the past 3 months. -OR Biopsy proven JXG, ECD, RDD, histiocytic sarcoma, or other histiocytic lesion (newly diagnosed or relapsed/refractory disease) with evaluable active disease. Performance Level: -Karnofsky ≥ 50% for patients > 16 years of age and Lansky ≥ 50% for patients ≤ 16 years of age. Adequate Hematologic Function Defined as  Prior and Concomitant Use of Drugs with CYP3A4 inducing/inhibiting activity: Patient taking strong inducers or inhibitors of CYP3A4 within 14 days prior to study enrollment, including but not limited to the following: erythromycin, clarithromycin, ketoconazole, azithromycin, itraconazole, grapefruit juice or St. John's wort Prior Therapy Restrictions Completion of previous chemotherapy, immunotherapy, radiotherapy, or targeted therapy for LCH (or other histiocytic disorder) at least 28 days (except where specified below) prior to study database registration, with resolution of all associated toxicity to ≤ Grade 1 prior to study enrollment (exception for alopecia and ototoxicity which do not need to be resolved ≤ Grade 1). Patients must have fully recovered from the acute toxic effects of all prior anti-cancer therapy and must meet the following minimum duration from prior anti-cancer directed therapy prior to enrollment. If after the required timeframe, the laboratory are met, the patient is considered to have recovered adequately Radiation therapy within the last 28 days Any prior treatment with Cobimetinib Treatment with a long-acting hematopoietic growth factor within 14 days prior to initiation of study drug or a short-acting hematopoietic growth factor within 7 days prior to database registration Treatment with hormonal therapy (except hormone replacement therapy or oral contraceptives), immunotherapy, biologic therapy, investigational therapy, or herbal cancer therapy within 28 days or < 5 half-lives, whichever is longer, prior to database registration Treatment with high-dose chemotherapy and stem-cell rescue (autologous stem cell transplant) or allogeneic stem cell transplant within 90 days prior to study database registration. 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 For patients with brain tumors (intracranial masses), use of anticoagulants within 7 days prior to study database registration Corticosteroid therapy <0.5 mg/kg/day averaged during the month prior to study database registration is permissible but must be discontinued fourteen (14) days prior to prior to study database registration. Patients with documented brain lesions receiving corticosteroids for management of cerebral edema must be on a stable dose for fourteen (14) days prior to study database registration Patient has received treatment with investigational therapy within 4 weeks prior to initiation of study drug