The present invention relates to compositions and methods for treating neoplasms in mammals, particularly humans. More particularly, the present invention provides for methods for preventing, treating or delaying neoplasm in a mammal using an ErbB-3 protein, a nucleic acid encoding an ErbB-3 protein or a functional fragment thereof. The present invention also provides for isolated nucleic acids encoding an extracellular domain of the ErbB-3 protein, or a functional fragment thereof, substantially purified extracellular domain of the ErbB-3 protein, or a functional fragment thereof and antibodies that bind to an epitope in an extracellular domain of the ErbB-3 protein, or a functional fragment thereof. The present invention further provides for pharmaceutical compositions and/or vaccines comprising the extracellular domain of the ErbB-3 protein, or a functional fragment thereof, or nucleic acids encoding and antibodies binding to such extracellular domain or functional fragments thereof.