Although oncogenes are involved in tumor development and progression, their activation during human ontogenesis is inseparably associated with normal fetal development. The c-erbB-2-encoded oncoprotein p185 (HER-2/neu) is overexpressed on fetal epithelial cells, in the placenta, and in several human carcinomas. In patients with p185-overexpressing tumors and in pregnant women at term, increased serum levels of a 105 kDa proteolytic breakdown product corresponding to the extracellular domain of oncoprotein p185 are detectable. Estrogens have been described to be potent inhibitors of p185 expression in human breast cancer cells and to influence also p105 serum levels in females. Regarding the significantly poorer prognosis of patients with c-erbB-2-positive tumors, we discuss common features and differences between c-erbB-2 oncoprotein overexpression in pregnancy and in carcinogenesis.