It is thought that autoantibody (aAb) production can be caused by (aberrant) protein targeting to the plasma surface of cells. We recently demonstrated the presence of the human cytochrome P450 enzyme CYP4Z1 on the plasma membrane of MCF-7 breast cancer cells and the detection of high titers of anti-CYP4Z1 aAbs in breast cancer patients, but not in healthy controls. In the present study we show that cells of the normal breast cell line MCF-10A do not display CYP4Z1 on their surface. By contrast, we detected CYP19A1 (aromatase) on the plasma membrane of both cell lines. Interestingly, the presence of CYPs on the cell surface did not correlate with their relative expression levels in these cell lines. Indirect ELISA experiments demonstrated the presence of anti-CYP19A1 aAbs in female breast cancer patient sera as well as in male and female controls, respectively; aAb titers in all three groups varied considerably and overall, the results obtained for each group were not significantly different from those of either of the other two groups. Based on these data we propose the hypothesis that CYP translocation to the plasma membrane, but not the intracellular expression level, is the crucial precondition for the generation of anti-CYP aAbs.