PAINTER: Polymorphism And INcidence of Toxicity in ERibulin Treatment

This study is primarily aimed at surveying the tolerability profile of Eribulin in an unselected population of patients with metastatic breast cancer in relation to toxicities already described in clinical trials, and neurotoxicity in particular. The secondary objectives of this trial include: * To study the relationship between specific genetic polymorphism and incidence and severity of peripheral neuropathy * To describe treatment efficacy in terms of duration of treatment and impact on survival. All toxicities will be collected and classified according to National Cancer Institute Common Terminology criteria for Adverse Events (NCI CTCAE) version 4.0 and monitored during all the treatment period and up to 30 days after therapy discontinuation. In particular, evaluation of incidence and outcome of any grade AEs already recorded in previous clinical trials will be collected, as follows: * asthenia/fatigue, * neutropenia, * alopecia, * nausea, * peripheral neuropathy * constipation Any other unexpected AEs shall be evaluated likewise. Patients must be followed for AEs until every ongoing Eribulin-related/unrelated toxicity and AE have been resolved, or the Investigator assesses them as "chronic" or "stable" or until the end of the trial, whichever comes first. For patients who will begin a new anticancer therapy after the last study drug administration, the AEs reporting period will end at the time the new treatment starts. For the determination of polymorphisms, a routine blood collection of two tubes with 3-5 ml of blood be performed. The sample can be collected at any time during the participant's first two treatment cycles. Blood will be collected in a Vacutainer containing ethylendiaminetetraacetic acid (EDTA). Immediately after blood collection, tubes have to be inverted (at least five times) and then stored at - 20° C.