Inclusion Criteria:

          -  Patients with relapsed/refractory B- or T-cell ALL

          -  Performance status =< 3 (Eastern Cooperative Oncology Group [ECOG] Scale)

          -  Total serum bilirubin =< 2 x upper limit of normal (ULN), unless due to Gilbert's
             syndrome, in which case patients are eligible as long as direct bilirubin =< 2 x ULN

          -  Alanine aminotransferase (ALT) =< 3 x ULN, unless due to disease involvement of the
             liver or hemolysis, in which case an ALT =< 10 x ULN is acceptable

          -  Aspartate aminotransferase (AST) =< 3 x ULN, unless due to disease involvement of the
             liver or hemolysis, in which case an AST =< 10 x ULN is acceptable

          -  Creatinine clearance >= 30 mL/min

          -  For females of childbearing potential, a negative pregnancy test must be documented
             within 1 week of starting treatment

          -  Female and male patients who are fertile must agree to use an effective form of
             contraception (birth control methods while on study, such as birth control pills or
             injections, intrauterine devices [IUDs]), or double-barrier methods (for example, a
             condom in combination with spermicide) with their sexual partners for 4 months after
             the end of treatment

          -  Signed informed consent

        Exclusion Criteria:

          -  Patients with Philadelphia chromosome-positive ALL or Burkitt leukemia

          -  Active serious infection not controlled by oral or intravenous antibiotics

          -  Active secondary malignancy other than skin cancer (e.g., basal cell carcinoma or
             squamous cell carcinoma) that in the investigator's opinion will shorten survival to
             less than 1 year

          -  Known hepatitis B or C infection, or known seropositivity for human immunodeficiency
             virus (HIV)

          -  Active grade III-V cardiac failure as defined by the New York Heart Association
             Criteria

          -  Patients with a cardiac ejection fraction (as measured by either multigated
             acquisition [MUGA] or echocardiogram) < 40%

          -  Received moderate or strong CYP3A inhibitors or strong CYP3A inducers within 7 days of
             starting venetoclax

          -  Consumed grapefruit, grapefruit products, Seville oranges, or star fruit within 3 days
             prior to starting venetoclax

          -  Prior history of treatment with venetoclax

          -  Treatment with any investigational antileukemic agents or chemotherapy agents in the
             last 7 days before study entry, unless full recovery from side effects has occurred or
             patient has rapidly progressive disease judged to be life-threatening by the
             investigator. Exception: Treatment with hydroxyurea and/or dexamethasone are allowed
             prior to study treatment, without window of exclusion

          -  Pregnant and lactating women will not be eligible; women of childbearing potential
             should have a negative pregnancy test prior to entering on the study and be willing to
             practice methods of contraception. Women do not have childbearing potential if they
             have had a hysterectomy or are postmenopausal without menses for 12 months. In
             addition, men enrolled on this study should understand the risks to any sexual partner
             of childbearing potential and should practice an effective method of birth control

          -  History of significant bleeding disorder unrelated to cancer, including: diagnosed
             congenital bleeding disorders (e.g., von Willebrand's disease); diagnosed acquired
             bleeding disorder within one year (e.g., acquired anti-factor VIII antibodies)