Dimethylfumarate (DMF) in Relapsed/Refractory CLL/SLL

Inclusion Criteria: * Clinical and phenotypic verification of B cell CLL/ SLL/ or MBL and measurable disease. * Relapsed or refractory disease * Previously treated with at least 1 regimen for CLL/SLL * Not appropriate or amenable to all approved therapies. * All patients must have progressed on or after B-cell receptor targeted kinase inhibitor (eg: ibrutinib, idelalisib, ACP-196, CC-292), unless there is a relative contraindication (eg: history of recent bleeding, history of atrial fibrillation, unacceptable high out-of-pocket cost despite patient assistance programs). * Patients with Del(17p) CLL must have progressed on or after BCL-2 inhibitor therapy (eg: venetoclax), unless there is a relative contraindication (eg: Creatinine clearance \< 50ml/min, or unable to monitor for TLS due to living remotely from the medical center, unacceptably high out-of-pocket cost). * Patients must have received a CD20-directed monoclonal antibody (eg: obinutuzumab, ofatumumab, rituximab), unless there is a relative contraindication (eg: history of hepatitis virus infection). * Has recovered from the toxic effects of prior therapy to their clinical baseline. * Women of childbearing potential must agree not to become pregnant for the duration of the study. * Both men and women must agree to use a barrier method of contraception for the duration of the study and until 8 weeks after the final dose. * Subjects must have at least one of the following indications for treatment: * Symptomatic or progressive splenomegaly; * Symptomatic lymph nodes, nodal clusters, or progressive lymphadenopathy; * Progressive anemia; * Progressive thrombocytopenia; * Weight loss \> 10% body weight over the preceding 6 month period; * Fatigue attributable to CLL; * Fever or night sweats for \> 2 weeks without evidence of infection; * Progressive lymphocytosis with an increase of \> 50% over a 2-month period or an anticipated doubling time of less than 12 months. * ECOG performance status of 0-2. * Adequate hematologic function * Adequate renal function * Adequate hepatic function Exclusion Criteria: * Pregnant or breast-feeding women will not be entered on this study. * Patients who are currently receiving another investigational agent are excluded. * Patients who have had chemotherapy (e.g., purine analogues, alkylating agents), radiation therapy, or participation in any investigational drug treatment within 4 weeks of initiation of DMF or at any time during the study. * Patients who have had prior (within 8 weeks of initiation of DMF) or concurrent antibody therapy directed against CLL (i.e. Rituxan and Campath) * Patients who have had tyrosine kinase inhibitor therapy (eg: ibrutinib or idelalisib) within 7 half lives (or 28 days, which ever is shorter) of initiation of DMF. * Current infection requiring parenteral antibiotics. * Active malignancy within the previous 2 years (other than completely resected non-melanoma skin cancer or carcinoma in situ). * Insufficient recovery from surgical-related trauma or wound healing. * Impaired cardiac function including any of the following: * Myocardial infarction within 6 months of starting study drug; * Other clinically significant heart disease