The goal of this retrospective observational study is to evaluate any possible association between plasma concentrations of ponatinib and its pharmacodynamics (efficacy/tolerability) in patients affected by chronic myeloid leukemia in chronic phase (CML-CP). In particular, the aims of the study will be: * primary aim: to investigate the relationships (if any) between plasma concentrations and activity/toxicity of ponatinib in a population of CML-CP patients enrolled in several Italian hematological centers; * secondary aim: to set up an algorithm aimed at helping physicians to improve drug dosing based on several variables (i.e., plasma drug concentrations, tolerability, molecular response to therapy). The study will enroll CML-CP patients who were exposed to ponatinib as second, third or fourth line of chemotherapy.