Three patients constituting a cohort will receive Clofarabine and will then be assessed for dose limiting toxicities (DLT) at Cycle 1. If none of these three patients develops DLT, the next cohort will be introduced. If one of them develops DLT, three new patients will be added to the cohort, so that six patients in total are included in the tolerability assessment. In this case, treatment of the next cohort is allowed only in the case the number of patients who develop DLT is still one in this six-patient cohort. If two of the six patients develop DLT, however, the tolerability is ruled out. However, if two DLTs are observed at the 20mg/m2/day dose cohort, new patients will be enrolled at cohort -1 15mg/m2/day; and if no more than one of six patients in the cohort develop DLT, it will be considered as the last cohort for this study.