- Tumor biopsy to research purposes. The purpose of this part of the study is to perform gene sequencing (gene tests) on each participant's cancer cells. The investigator will request your permission to obtain samples of one of these areas: breast, skin, chest, lymph node, soft tissue, bone, or liver for research purposes. The chosen area will be the one that the treating provider considers appropriate. Alternatively we will be asking the patient to use fluid from a thoracentesis or paracentesis. If the patients are having a biopsy/thoracentesis or paracentesis as clinical care, then some tissue will be removed from that sample for research. If a biopsy biopsy is not be needed for clinical care then it will be performed for research purposes. This only will be performed to the participants enrolled after determination the safest dose of eribulin. It is planned at baseline. The biopsy after tumor progression is optional.