Study of bb21217 in Multiple Myeloma

Inclusion Criteria: * ≥18 years of age at the time of signing informed consent * Eastern Cooperative Oncology Group (ECOG) performance status 0 or 1 * Diagnosis of MM with relapsed or refractory disease and have had at least 3 different prior lines of therapy with previous exposure to PI, IMiDs, and a CD38 antibody. Have undergone at least 2 consecutive cycles of treatment for each therapy, unless PD was the best response to the therapy. Refractory to their last line of therapy. * Subjects must have measurable disease Exclusion Criteria: * Subjects with known central nervous system disease * Inadequate hepatic function * Inadequate renal function * Inadequate bone marrow function * Presence of active infection within 72 hours * Subjects with a history of stroke, unstable angina, myocardial infarction, or ventricular arrhythmia requiring medication or mechanical control * Significant co-morbid condition or disease which in the judgment of the Investigator would place the subject at undue risk or interfere with the study; examples include, but are not limited to, cirrhotic liver disease, sepsis, recent significant traumatic injury, and other conditions * Known human immunodeficiency virus (HIV) positivity * Known hepatitis A virus (HAV), hepatitis B virus (HBV) or hepatitis C virus (HCV) positivity with evidence of ongoing infection. * Pregnant or lactating women * Previous history of an allogeneic bone marrow transplantation, treatment with any gene therapy based therapeutic for cancer, or BCMA-targeted therapy * Inadequate pulmonary function defined as oxygen saturation (SaO2) \<92% on room air * Subjects who have a history of plasma cell leukemia, active plasma cell leukemia, Waldenstrom's macroglobulinemia, POEMS, or clinically significant amyloidosis