Inclusion Criteria:

          -  Phase I: Patients must have histologically confirmed breast cancer (metastatic breast
             cancer [MBC]) that is human epidermal growth factor receptor 2 (HER2/neu) negative (as
             determined by local pathology or reference laboratory), and have disease that is
             metastatic (stage IV [TxNxM1]) or locally advanced and not amenable to potentially
             curative surgical resection (eg, clinical stage IIIB-C)

          -  HER2/neu negative disease (performed on primary tumor and/or metastatic lesion using
             commercially available/approved assay in local institutional or reference laboratory),
             according to American Society of Clinical Oncology (ASCO)/College of American
             Pathologists (CAP) guidelines

          -  National Comprehensive Cancer Network (NCCN) guidelines recommend for metastatic
             breast cancer "…biopsy documentation of first recurrence, if possible, and
             determination of hormone receptor status (estrogen receptor [ER] and progesterone
             receptor [PR]) and HER2 status…."; therefore, histologic and/or cytologic confirmation
             of metastatic disease is encouraged whenever feasible, but not required; in some
             circumstances, histologic confirmation may not be feasible (eg, bone metastases not
             amenable to biopsy and elevated cancer antigen [CA]27-29 tumor marker); for patients
             who have had histologic confirmation of metastatic disease, it is required that the
             biopsy confirm that the metastatic tumor is ER and/or PR positive, and HER2/neu
             negative; for patients in whom biopsy confirmation of metastatic disease is not
             feasible, it is required that the primary tumor be ER and/or PR-positive and HER2/neu
             negative

          -  Measurable disease (Response Evaluation Criteria in Solid Tumors [RECIST] 1.1) or
             non-measurable disease, with measurement obtained within 4 weeks of registration

          -  Phase I: Patients must have received at least one prior chemotherapy regimen for
             metastatic disease; patients with deleterious germ line mutations in breast cancer
             (BRCA)1 or BRCA2 are not required to have received prior chemotherapy for metastatic
             disease

          -  Patients must have had progressive disease after at least one line of endocrine
             therapy for metastatic disease (includes relapse while receiving endocrine therapy);
             there should be at least 1 week interval between the last endocrine treatment for an
             aromatase inhibitor and at least 2 weeks for tamoxifen or fulvestrant

          -  Eastern Cooperative Oncology Group (ECOG) performance status 0, 1, or 2 (Karnofsky >=
             60%)

          -  Leukocytes >= 3,000/mcL

          -  Absolute neutrophil count >= 1,500/mcL

          -  Platelets >= 100,000/mcL

          -  Hemoglobin >= 9 g/dl (per manufacturer recommendation)

          -  Total bilirubin within normal institutional limits (unless isolated indirect
             hyperbilirubinemia due to Gilbert's disease)

          -  Aspartate aminotransferase (AST) (serum glutamic oxaloacetic transaminase
             [SGOT])/alanine aminotransferase (ALT) (serum glutamate pyruvate transaminase [SGPT])
             =< 2.5 × institutional upper limit of normal

          -  Creatinine within normal institutional limits OR creatinine clearance >= 60
             mL/min/1.73 m^2 for patients with creatinine levels above institutional normal

          -  Patients with a history of brain metastases are eligible if they have been treated
             with radiation and have stable brain metastases at least 3 months after radiation and
             must also be off steroids

          -  Patients must be able to swallow whole capsules and tolerate oral medications

          -  Women of child-bearing potential and men must agree to use adequate contraception
             (hormonal or barrier method of birth control; abstinence) prior to study entry and for
             the duration of study participation; being not of childbearing potential is defined
             as: (1) prior hysterectomy, or (2) no menstrual period for at least 24 months; should
             a woman become pregnant or suspect she is pregnant while participating in this study,
             she should inform her treating physician immediately

          -  Ability to understand and the willingness to sign a written informed consent document

        Exclusion Criteria:

          -  Patients who have radiotherapy within 3 weeks prior to entering the study or those who
             have not recovered from adverse events due to systemic agents administered more than 3
             weeks earlier

          -  Patients may not be receiving any other investigational agents

          -  Patients with known brain metastases with active symptoms or requiring anticonvulsive
             medications, or steroids should be excluded from this clinical trial

          -  History of allergic reactions attributed to compounds of similar chemical or biologic
             composition to veliparib (ABT-888) or cyclophosphamide used in the study

          -  Evidence of complete or partial bowel obstruction or other unable to take oral
             medications

          -  Patients with malabsorption syndrome or other condition that would interfere with
             intestinal absorption

          -  Patients unable to swallow whole capsules

          -  Uncontrolled intercurrent illness including, but not limited to, ongoing or active
             infection, symptomatic congestive heart failure, unstable angina pectoris, cardiac
             arrhythmia, or psychiatric illness/social situations that would limit compliance with
             study requirements

          -  Pregnant (positive pregnancy test) or lactating women will be excluded from the study;
             also, unwillingness to use effective means of contraception in subjects with
             child-bearing potential will be excluded from the study; women of child-bearing
             potential must use two forms of contraception (i.e., barrier contraception and one
             other method of contraception) at least 4 weeks prior to study entry, for the duration
             of study participation

          -  Patients with active severe infection; known infection with human immunodeficiency
             virus (HIV), hepatitis B virus, hepatitis C virus, or severe concurrent illness will
             be excluded from the study; HIV-positive patients on combination antiretroviral
             therapy are ineligible

          -  Patients with a history of seizure disorder requiring antiepileptics who have had a
             seizure episode within the last 6 months

          -  Prior treatment with veliparib (ABT-888) or other PARP inhibitors (e.g., olaparib)