The EGF family of receptor tyrosine kinases (a.k.a. ERBB receptors) mediate the effects of the epidermal growth factor (EGF) family including Neuregulin-1β (NRG). NRG and ERBB1-4 are critical for cardiac development and maintenance of the adult heart. Current understanding of the role of EGF/NRG/ERBB signaling in the cardiovascular system is rapidly evolving due to recent findings in non-myocyte cell populations. This study is examining a population of progenitor cells in the adult human heart that responds to EGF and NRG. Subjects are enrolled who are scheduled to undergo heart surgery and are willing to allow for a small biopsy to be taken from their hearts during surgery. Biopsies are taken to the laboratory where cells are separated and analyzed by flow cytometry and grown in cell culture to understand how their biology is regulated by NRG and EGF.