Inclusion Criteria:

          -  Patients with confirmed diagnosis of acute leukemia with MLL rearrangement according
             to the 2008 WHO Classification

          -  Patients with MLL-rearranged leukemia who are refractory to standard induction therapy
             and not immediate candidates for allogeneic HSCT (bridge to transplant is allowed)

          -  Patients with MLL-rearranged leukemia who relapsed after standard first-line treatment
             and are not immediate candidates for allogeneic HSCT (bridge to transplant is allowed)

          -  Patients with newly diagnosed MLL-rearranged leukemia who are not eligible for
             intensive first-line therapy

          -  Genetic/histologic/immunohistologic assessment in one of the central laboratories

          -  Age ≥ 18 years, no upper age limit

          -  WHO performance status of ≤ 2

          -  No prior chemotherapy two weeks before study entry except hydroxyurea to control
             hyperleukocytosis

          -  Non-pregnant and non-nursing. Women of child-bearing potential must have a negative
             serum or urine pregnancy test with a sensitivity of at least 25 mIU/mL within 72 hours
             prior to registration (WOCBP is defined as a sexually active mature woman who has not
             undergone a hysterectomy or who has had menses at any time in the preceding 24
             months).

          -  Female patients in the reproductive age and male patients must agree to avoid getting
             pregnant or to father a child while on therapy and for three months after the last
             dose of therapy.

          -  Women of child-bearing potential must either commit to continued abstinence from
             heterosexual intercourse or begin one acceptable method of birth control (IUD, tubal
             ligation, or partner's vasectomy). Hormonal contraception is an inadequate method of
             birth control.

          -  Men must agree not to father a child and must use a latex condom during any sexual
             contact with WOCBP while receiving therapy and for three months after therapy is
             stopped, even if they have undergone successful vasectomy.

          -  Signed written informed consent

        Exclusion Criteria:

          -  Prior treatment with palbociclib

          -  Performance status > 2 according to WHO criteria

          -  Organ insufficiency: creatinine > 1.5 x upper normal serum level; bilirubin, AST, or
             AP > 2.5 x upper normal serum level; heart failure NYHA III/IV; uncontrolled
             hypertension; unstable angina; serious cardiac arrhythmia; severe obstructive or
             restrictive ventilation disorder

          -  Uncontrolled infection

          -  Patients with a "currently active" second malignancy other than non-melanoma skin
             cancer. Patients are not considered to have a "currently active" malignancy if they
             have completed therapy and are considered by their physician to be at less than 30%
             risk of relapse within one year.

          -  Severe neurologic or psychiatric disorder interfering with ability of giving informed
             consent

          -  Known or suspected active alcohol or drug abuse

          -  Known positivity for HIV, active HAV, HBV, or HCV infection

          -  Bleeding disorder unrelated to leukemia

          -  Uncontrolled CNS involvement (treatment for CNS-involvement prior to inclusion is
             allowed)

          -  QTc > 470 msec (based on the mean value of triplicate ECGs), family or personal
             history of long or short QT syndrome, Brugada syndrome, or known history of QTc
             prolongation or Torsade de Pointes

          -  Uncontrolled electrolyte disorders that can aggravate the effects of a QTc-prolonging
             drug (e.g., hypocalcemia, hypokalemia, hypomagnesemia)

          -  No consent for registration, storage, and processing of individual disease
             characteristics, information on the course of the disease, and information obtained
             from the family physician and/or other physicians involved in the treatment of the
             patient about study participation

          -  No consent for biobanking