A Study Evaluating Venetoclax Alone and in Combination With Azacitidine in Participants With Relapsed/Refractory Myelodysplastic Syndromes (MDS)

* Subjects who have relapsed or refractory MDS. * Subject enrolled in venetoclax monotherapy must have documented failure of prior therapy with a hypomethylating agent (HMA). HMA-failure is defined as: 1. Relapse after initial complete or partial response or hematological improvement after at least 4 cycles of azacitidine or at least 4 cycles of decitabine within the last 5 years, OR 2. Failure to achieve complete or partial response or hematological improvement after at least 4 cycles of azacitidine or at least 4 cycles of decitabine within the last 5 years * Subjects must have presence of \< 20% bone marrow blasts per bone marrow biopsy/aspirate at screening. * Subject is not a candidate to undergo allogenic hematopoietic stem cell transplantation (HSCT). * Subject must have an Eastern Cooperative Oncology Group (ECOG) performance score of ≤2. * Subject must have adequate hematologic, renal, and hepatic function.

* Subject has received prior therapy with a BH3 mimetic. * Subject has MDS evolving from a pre-existing myeloproliferative neoplasm (MPN). * Subject has MDS/MPN including chronic myelomonocytic leukemia (CMML), atypical chronic myeloid leukemia (CML), juvenile myelomonocytic leukemia (JMML) and unclassifiable MDS/MPN. * Subject has received allogeneic HSCT or solid organ transplantation. * Subject has received a live attenuated vaccine within 4 weeks prior to the first dose of study drug. * Subject is pregnant or breastfeeding.