Inclusion Criteria:

* Participants must have MDS or MDS/myeloproliferative overlap disorder with potential sensitivity to HMA therapy, defined as prior published evidence of response to HMA

* Myelodysplastic Syndromes:

* As classified by hematopathology review of WHO categories, myelodysplastic/myeloproliferative neoplasm unclassifiable, refractory anemia with ring sideroblasts and thrombocytosis, refractory cytopenia with unilineage dysplasia (RCUD), refractory anemia with ring sideroblasts (RARS), refractory cytopenia with multi-lineage dysplasia (RCMD), refractory anemia with excess blasts (RAEB), myelodysplastic syndrome with isolated del(5q), myelodysplastic syndrome unclassifiable (MDS-U).
* Participant with MDS who are IPSS-R high and very high risk or IPSS intermediate 2 risk and higher are excluded given proven overall survival benefit in higher risk MDS from AZA-001 with this treatment
* Myelodysplastic/myeloproliferative neoplasm overlap disorders ---MDS/MPN crossover syndromes with limited evidence of extramedullary hematopoiesis (may not have palpable splenomegaly) and reticulin fibrosis of grade 1 or less without evidence of progression to accelerated phase. These may include but may not be limited to RARS-T, CMML, Atypical CML (BCR-ABL negative), and MDS/MPN NOS
* Indication for HMA therapy: Symptomatic anemia OR thrombocytopenia with a platelet count of \<100 x 109/L OR transfusion dependence for red-cells OR transfusion dependence for platelets OR absolute neutrophil count \< 1.0 x 109/L

--Participants with lower risk MDS must have must have failed or have contraindications to available therapies (e.g. lenalidomide, epoetin if indicated for symptomatic anemia and/or transfusion dependence of red cells) known to be effective for treatment of their disease
* Participants must have performance status of 60% or greater by Karnofsky Performance Status (KPS)
* Must have adequate end organ function defined as:

* AST and ALT \< 3× the upper limit of normal (ULN)
* Bilirubin ≤ 1.5× the ULN. If elevated bilirubin is due to impaired conjugation (e.g Gilbert's disease or concomitant medication) or disease related hemolysis, then direct bilirubin ≤ 1.5× the ULN
* As azacitidine and decitabine have little renal metabolism, and have proven safety even in dialysis participants, renal function is not an inclusion or exclusion criteria
* Subjects must have the ability to understand and the willingness to sign a written informed consent document and complete study related procedures.

Exclusion Criteria:

* MDS with IPSS-R high or very high risk, or IPSS intermediate-2 or high risk disease
* Prior Treatment with azacitidine, decitabine or investigational HMA therapy with overlapping mechanism of action (e.g. guadecitibine)
* No other disease directed therapy, save for hydroxyurea, including experimental or investigational drug therapy for 14 days prior to study entry.
* Toxicity (grade 2 or higher) from prior therapies including chemotherapy, targeted therapy, immunotherapy, experimental therapy, radiation or surgery must be resolved to grade 1 or less.
* Currently pregnant or breast-feeding. Females of child bearing (FOCBP) potential must have negative serum pregnancy test within 72 hours from treatment start. (NOTE: FOCBP is any biologic female, regardless of sexual or gender orientation, having undergone tubal ligation, or remaining celibate by choice, who has not undergone a documented hysterectomy or bilateral oophorectomy or has had a menses any time in the preceding 12 months (therefore not naturally post-menopausal for \> 12 months)
* Uncontrolled intercurrent illness that could limit life expectancy or ability to complete study correlates. This includes, but is not limited to:

* Ongoing or active infection. As participants with MDS and MDS/MPNs are prone to infections, if participants are actively being treated with appropriate antibiotics or antifungal therapy with clinical evidence of infection control, then they will be considered eligible for study.
* Uncontrolled concurrent malignancy
* Congestive heart failure of NYHA class III/IV. Participants with compensated heart failure are permitted.
* Unstable angina pectoris
* New or unstable cardiac arrhythmia. Stable or controlled arrhythmias are permitted
* Decompensated liver cirrhosis (Child-Pugh score ≥12 or a MELD score ≥21)
* Psychiatric illness/social situations that would limit compliance with study requirements.
* Any other prior or ongoing condition, in the opinion of the investigator, that could adversely affect the safety of the participant or impair the assessment of study results.
* WOCBP and males that are unwilling to agree to use dual contraceptive measures (i.e., hormonal or barrier method of birth control; abstinence, condom) prior to study entry and for the duration of study participation. Should a female subject become pregnant or suspect she is pregnant while participating in this study, she should inform the treating physician immediately
* Sexually active male who is unwilling to use a condom when engaging in any sexual contact with a female with child-bearing potential, beginning at the screening visit and continuing until 4 weeks after taking the last dose of 5AZA-alt-DEC.
* Participants with known active HIV infection, as this will further increase the risk for opportunistic infections. However, participants with chronic HIV with undetectable viral load by PCR, without opportunistic infection, and on a stable regimen of antiretroviral therapy would be eligible.
* Known allergy or hypersensitivity to any component of azacitidine or decitabine formulations