Inclusion Criteria: 1. Age ≥ 18 years. 2. Signed written informed consent prior to any study procedure. 3. Relapsed and/or refractory multiple myeloma (MM). 1. Subjects must have documented progressive disease as per International Myeloma Working Group (IMWG) criteria during or within 12 months of completing treatment with the last anti-myeloma treatment regimen before study entry. Also, subjects with confirmed progressive disease within 6 months prior to start of Screening and who are refractory (or non-responsive) to their most recent anti-myeloma treatment regimen afterwards will be also eligible. 2. Part A and Part B Cohort A: Subjects must have confirmed at least 3 prior antimyeloma treatment regimens. 3. Part B Cohort B only: Subjects must have received at least 1 but no greater than 3 prior antimyeloma treatment regimens, including a proteasome inhibitor and immunomodulatory agent. 4. Subjects must have previously received all of the following therapies: i) Autologous stem cell transplant ii) A regimen that included an immunomodulatory agent (eg, thalidomide, lenalidomide, pomalidomide) and a proteasome inhibitor (eg, bortezomib, carfilzomib, ixazomib), either alone or combination iii) Anti-CD38 (eg, daratumumab), either alone or combination Subjects in Cohort B do not require prior anti-CD38 antibody therapy. 4. Measurable disease 5. Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1 6. Adequate organ function Exclusion Criteria: 1. Known active or history of central nervous system (CNS) involvement of MM 2. Active or history of plasma cell leukemia, Waldenstrom's macroglobulinemia, POEMS (polyneuropathy, organomegaly, endocrinopathy, monoclonal protein, skin changes) syndrome, or clinically significant amyloidosis 3. Prior treatment with CAR T-cell or another genetically modified T-cell therapy 4. Part A and Part B Cohort A only: Prior treatment with investigational therapy directed at BCMA 5. Uncontrolled or active infection 6. Active autoimmune disease requiring immunosuppressive therapy 7. History or presence of clinically significant CNS pathology such as seizure disorder, aphasia, stroke, severe brain injuries, dementia, Parkinson's disease, cerebellar disease, organic brain syndrome, or psychosis