Additional Study Information: The purpose of this study is to test a new medicine to prevent liver venoocclusive disease (VOD) in patients undergoing hematopoietic stem cell transplant (HSCT) who are at high risk or very high risk for liver VOD following HSCT. Liver VOD can be a complication of bone marrow and/or stem cell transplant,which may be life threatening. The condition is a possible side effect of the chemotherapy or conditioning regimen you receive to help treat your cancer. VOD is characterized by damage to blood vessels in the liver and surrounding liver cells, leading to pain, fluid retention and abnormal liver function. This research study is evaluating a medicine called defibrotide for possible prevention of liver VOD (those who do not have a VOD diagnosis) in adult and pediatric patients.