Inclusion Criteria:

          1. Female, age at least 18 years

          2. ECOG performance status of <2

          3. Written voluntary informed consent

          4. Histologically diagnosed Uterine Serous Carcinoma.

          5. Patient must agree to undergo Foundation One testing.

          6. Patient diagnosed with advanced stage USC including stage III, stage IV, or
             platinum-sensitive recurrent USC

          7. If recurrent USC, patient must have platinum sensitive disease after initial
             treatment; defined as achieving a response (CR or PR) and disease progression >6
             months after completion of their last dose of platinum chemotherapy.

          8. Patients eligible if receiving 1st or 2nd line chemotherapy for recurrence.

          9. The patient must have achieved a partial, stable, or complete tumor response following
             the last chemotherapy (minimal of 3 cycles) regimen of physician choice chemotherapy
             indicating partial, stable, complete tumor response.

         10. Patients must receive Niraparib maintenance within 12 weeks after completion of their
             final dose of chemotherapy regimen or within 14 weeks if received radiation therapy.
             CT Chest/Abd/Pelvis will be performed within 28 days of starting Niraparib.

         11. Lesions can be non-measurable or measurable by RECIST 1.1 criteria.

         12. Adequate organ function, defined as:

               1. Absolute neutrophil count ≥ 1,500/μL

               2. Platelets ≥ 100,000/μL

               3. Hemoglobin ≥ 9 g/dL

               4. Serum creatinine ≤ 1.5 x upper limit of normal (ULN) or calculated creatinine
                  clearance ≥ 30 mL/min using the Cockcroft-Gault equation

               5. Total bilirubin ≤ 1.5 x ULN (≤2.0 in patients with known Gilberts syndrome) OR
                  direct bilirubin ≤ 1 x ULN

               6. Aspartate aminotransferase and alanine aminotransferase ≤ 2.5 x ULN unless liver
                  metastases are present, in which case they must be ≤ 5 x ULN

         13. Participant receiving corticosteroids may continue as long as their dose is stable for
             least 4 weeks prior to initiating protocol therapy.

         14. Participant must agree to not donate blood during the study or for 90 days after the
             last dose of study treatment.

         15. Female participant has a negative urine or serum pregnancy test within 7 days prior to
             taking study treatment if of childbearing potential and agrees to abstain from
             activities that could result in pregnancy from screening through 180 days after the
             last dose of study treatment, or is of non childbearing potential. Non childbearing
             potential is defined as follows (by other than medical reasons):

               1. ≥45 years of age and has not had menses for >1 year Patients who have been
                  amenorrhoeic for <2 years without history of a hysterectomy and oophorectomy must
                  have a follicle stimulating hormone value in the postmenopausal range upon
                  screening evaluation Post-hysterectomy, post-bilateral oophorectomy, or
                  post-tubal ligation. Documented hysterectomy or oophorectomy must be confirmed
                  with medical records of the actual procedure or confirmed by an ultrasound. Tubal
                  ligation must be confirmed with medical records of the actual procedure,
                  otherwise the patient must be willing to use 2 adequate barrier methods
                  throughout the study, starting with the screening visit through 180 days after
                  the last dose of study treatment. See Section 6.4 for a list of acceptable birth
                  control methods. Information must be captured appropriately within the site's
                  source documents. Note: Abstinence is acceptable if this is the established and
                  preferred contraception for the patient.

               2. Participant must agree to not breastfeed during the study or for 180 days after
                  the last dose of study treatment.

               3. Able to take oral medications.

        Exclusion Criteria:

          -  1. Participant must not be simultaneously enrolled in any interventional clinical
             trial

             2. Drainage of ascites during the last 2 cycles of last chemotherapy

             3. Radiotherapy was given within 2 weeks encompassing >20% of the bone marrow or any
             radiation therapy within one week prior to Day 1 of protocol therapy. Participant must
             not have received investigational therapy ≤ 4 weeks, or within a time interval less
             than at least 5 half-lives of the investigational agent, whichever is shorter, prior
             to initiating protocol therapy.

             4. Persistent >Grade 2 anemia, neutropenia, or thrombocytopenia from prior cancer
             therapy, that has persisted > 4 weeks and was related to the most recent treatment.

             5. Symptomatic uncontrolled brain or leptomeningeal metastases.

             6. Known hypersensitivity to the components of Niraparib

             7. Major surgery within 3 weeks of starting the study or patient has not recovered
             from any effects of any major surgery

             8. Diagnosis, detection, or treatment of invasive cancer other than uterine cancer </=
             2 years prior to study enrollment (except basal or squamous cell carcinoma of the skin
             that has been definitively treated)

             9. Patient considered a poor medical risk due to serious, uncontrolled medical
             disorder, non-malignant systemic disease or active uncontrolled infection.

             10. Patients must not have received a transfusion within 4 weeks of the first dose of
             study treatment

             11. Participant must not have received colony stimulating factors (e.g., granulocyte
             colony-stimulating factor, granulocyte macrophage colony stimulating factor, or
             recombinant erythropoietin) within 4 weeks prior initiating protocol therapy.

             12. Participant must not have any known history of myelodysplastic syndrome (MDS) or
             acute myeloid leukemia (AML)

             13. Immunocompromised patients (splenectomy patients are allowed)

             14. Patients with known active hepatitis disease

             15. Prior treatment with a known PARP inhibitor

             16. Patients noted to have MSI-H mutational burden.