Inclusion Criteria:

          -  Patients must have the ability to understand and the willingness to signed a written
             informed consent document.

          -  Patients must have histologically or cytologically confirmed CRPC or endometrial
             cancer that is metastatic. Evidence of disease progression on a prior therapy is not
             required.

          -  Patients must have at least one lesion that is amenable to biopsy and the treating
             physician must deem this safe.

          -  Patient must be willing to undergo two mandatory research-only biopsies.

          -  Prostate cancer patients:

               -  Patients must be surgically or medically castrated, with serum testosterone
                  levels ≤ 50 ng/mL Patients being treated with Gonadotropin-releasing hormone
                  (GnRH) agonists must have such therapy continued throughout the study.

               -  Patients should have received at least one androgen receptor (AR)-targeted
                  therapy with abiraterone acetate or enzalutamide. Multiple lines of prior
                  AR-targeted therapy and chemotherapy are permitted. A washout of two weeks from
                  prior endocrine therapy (other than GnRH agonist); four weeks washout period from
                  prior cytotoxic chemotherapy or other anticancer agents is required (prior to day
                  1 of study therapy); six weeks washout period to allow for anti-androgen
                  withdrawal for patients managed with antiandrogen therapy such as bicalutamide.

          -  Endometrial cancer patients:

               -  An unlimited number of prior hormonal and/or chemotherapy regimens are permitted.
                  A washout of two weeks from prior endocrine therapy (other than GnRH agonist) and
                  four weeks from prior cytotoxic chemotherapy or other anticancer agents is
                  required (prior to day 1 of study therapy).

          -  At least 5 days should have elapsed since any non-study related minor surgical
             procedure and at least 21 days since any major surgical procedure prior to the first
             dose of rucaparib and the first dose of nivolumab.

          -  Must have an ability to swallow pills or capsules. Patients should have no current
             clinical evidence of bowel obstruction.

          -  Age must be ≥ 18 years.

          -  Eastern Cooperative Oncology Group (ECOG) performance status must be ≤ 1

          -  Patients must have normal hepatic, renal and marrow function as defined below:

               -  hemoglobin > 10 g/dL

               -  leukocytes ≥ 3,000/mcL

               -  absolute neutrophil count ≥ 1,500/mcL

               -  platelets ≥ 150,000/mcL

               -  total bilirubin within normal institutional limits

               -  alanine aminotransferase (ALT) and aspartate aminotransferase (AST) ≤ 1.5 ×
                  institutional upper limit of normal

               -  creatinine within normal institutional limits OR creatinine clearance ≥ 60
                  mL/min/1.73 m2 for patients with creatinine levels above institutional normal.

          -  Prior palliative radiotherapy must have been completed at least 2 weeks prior to first
             dose of study drug. Subjects with symptomatic tumor lesions at baseline that may
             require palliative radiotherapy within 4 weeks of first dose of study drug are
             strongly encouraged to receive palliative radiotherapy prior to enrollment.

          -  Reproductive Status: Rucaparib caused post-implantation loss (100% early resorptions)
             at all doses administered in an embryo-fetal development study. Based on its mechanism
             of action, nivolumab can cause fetal harm when administered to a pregnant woman.
             Pregnant women are therefore not eligible for this study.

               -  Women of child-bearing potential (WOCBP) must have a negative serum pregnancy
                  test result less than 3 days prior to administration of the first dose of
                  rucaparib.

               -  WOCBP must not be considering getting pregnant during the study.

               -  WOCBP and their male partners must agree to use a highly effective, reliable form
                  of contraception during treatment; for 6 months following the last dose of
                  rucaparib; and for at least 5 months following the last dose of nivolumab.

               -  Men who are sexually active with WOCBP must agree to use a highly effective,
                  reliable form of contraception during treatment; 6 months following the last dose
                  of rucaparib; and for a period of 7 months after the last dose of nivolumab .

               -  In addition to the above methods of contraception, use of a condom by male
                  patients is recommended to prevent transfer of drug through semen.

        Exclusion Criteria:

          -  Patients with active, known, or suspected autoimmune disease. Subjects with vitiligo,
             type I diabetes, mellitus, residual hypothyroidism due to autoimmune condition only
             requiring hormone replacement, psoriasis not requiring systemic treatment, childhood
             asthma that is not currently active, or conditions not expected to recur in the
             absence of an external trigger are permitted to enroll.

          -  Patients with a condition requiring systemic treatment with either corticosteroids (>
             10 mg daily prednisone equivalents) or other immunosuppressive medications within 14
             days of study drug administration except for adrenal replacement steroid doses > 10 mg
             daily prednisone equivalent in the absence of active autoimmune disease. Note:
             Treatment with a short course of steroids (< 5 days) up to 7 days prior to initiating
             study drug is permitted.

          -  Patients who are receiving any other investigational agents.

          -  Prior exposure to PD-1 or PD-L1 inhibitors, other immune checkpoint inhibitors (e.g.
             anti-LAG-3, and anti-CTLA-4 antibodies), or PARP inhibitors.

          -  Patients with a "currently active" second invasive malignancy other than non-melanoma
             skin cancers. Patients are not considered to have a "currently active" malignancy if
             they have completed therapy and have been free of disease for ≥ 1 years.

          -  Patients with known and untreated or progressing brain metastases are excluded from
             this clinical trial because of their poor prognosis and because they often develop
             progressive neurologic dysfunction that would confound the evaluation of neurologic
             and other adverse events. Patients whose brain metastases have been treated with
             surgery and/or radiotherapy and are without evidence of progression on scan for at
             least 4 weeks, and are off steroids or antiseizure medications, will be eligible .

          -  Patients with symptomatic or impending spinal cord compression are not eligible unless
             appropriately treated.

          -  History of allergic reactions attributed to compounds of similar chemical or biologic
             composition to nivolumab or rucaparib.

          -  Based on in vitro CYP interaction studies, caution should be used for concomitant
             medications with a narrow therapeutic window that are substrates of CYP2C19, CYP2C9,
             and/or CYP3A. Selection of an alternate concomitant medication is recommended. Caution
             should also be exercised for concomitant use of certain statin drugs (e.g.
             rosuvastatin and fluvastatin) due to potential increase in exposure from inhibition of
             BCRP and CYP2C9. An updated list of clinically relevant P450 drug interactions (e.g:
             Flockhart Table http://medicine.iupui.edu/clinpharm/ddis/main-table/) should be
             reviewed while screening patients for study.

               -  Patients taking warfarin should have international normalized ration (INR)
                  monitored regularly according to standard institutional practices

               -  Because rucaparib is a moderate inhibitor of P-gp in vitro, caution should be
                  exercised for patients receiving rucaparib and requiring concomitant medication
                  with digoxin. Patients taking digoxin should have their digoxin levels monitored
                  after starting rucaparib and then regularly per standard clinical practice.

          -  Patients on parenteral nutrition are not eligible. Patients must not have a
             pre-existing duodenal stent or any gastrointestinal disorder or defect that would, in
             the opinion of the treating investigator, interfere with absorption of rucaparib.

          -  Uncontrolled intercurrent illness including, but not limited to, requirement for
             oxygen therapy, ongoing or active infection other than minor urinary tract infection,
             symptomatic congestive heart failure, unstable angina pectoris, cardiac arrhythmia, or
             psychiatric illness/social situations that would limit compliance with study
             requirements.

          -  Known history of chronic hepatitis B or C as evidenced by:

               -  Positive test for hepatitis B surface antigen

               -  Positive test for qualitative hepatitis C viral load (by polymerase chain
                  reaction [PCR])

               -  Note: Subjects with positive hepatitis C antibody and negative quantitative
                  hepatitis C by polymerase chain reaction (PCR) are eligible.

          -  Human Immunodeficiency Virus (HIV)-positive patients on combination antiretroviral
             therapy are ineligible because of the potential for pharmacokinetic interactions with
             rucaparib. In addition, these patients are at increased risk of lethal infections when
             treated with marrow-suppressive therapy.

          -  Prior organ allograft or allogeneic bone marrow transplantation.

          -  Adverse effect of prior therapy not improved to Common Terminology Criteria for
             Adverse Events (CTCAE) Grade 1 or below with the exception of alopecia or lymphopenia.
             Ongoing Grade 2 non-hematologic toxicity (e.g. neuropathy) related to most recent
             treatment regimen may be permitted with prior advanced approval from the Lead
             Principal Investigator.

          -  Initiated denosumab or bisphosphonate therapy or adjusted denosumab or bisphosphonate
             dose/regimen within 4 weeks prior to first dose of rucaparib. Patients on a stable
             denosumab or bisphosphonate regimen are eligible and may continue treatment.

          -  Evidence or history of active or latent tuberculosis infection including purified
             protein derivative (PPD) recently converted to positive.

          -  Use of non-oncology vaccines containing live virus for prevention of infectious
             diseases within 12 weeks prior to study drug. The use of inactivated seasonal
             influenza vaccines, e.g., Fluzone®, will be permitted on study without restriction.