The new drug entinostat is a selective histone deacetylase inhibitor with a promising future, since it seems to be a new treatment option for those patients with breast cancer, where the standard endocrine therapy is not effective. Entinostat has proven to be able to convert estrogen receptor negative tumors into estrogen receptor positive tumors and by that made the tumor succeptable to endocrine treatment. Entinostat given together with an aromatase inhibitor has shown to slow down the progression in estrogen receptor-positive breast cancer and furthermore it is very likely that entinostat can reduce the formation of new metastasis.