In breast cancer the growth factor receptor erbB2/neu is often overexpressed and leads to uncontrolled proliferation even in the absence of ligand. If individual copies of the protein are isolated, they do not signal in the absence of ligand. What does this imply about how erbB2/neu functions? (Refer to our discussion from class of what can go wrong with signaling pathways to cause cancer.)
a) erbB2/neu requires ligand binding for activation.
b) erbB2/neu is constitutively active.
c) erbB2/neu functions as a tumor suppressor.
d) erbB2/neu is inactive in breast cancer.