The present invention relates to the discovery of soluble isoforms of an Epidermal Growth Factor Receptor, or sErbB1 variants, the provision of the sequences of nucleic acids encoding these isoforms, purified recombinant proteins, novel antibodies specific for these isoforms, and the use of immunoassay and other protein assay techniques to measure the concentration of these protein isoforms in a patient biological sample in the femtomolar range. The present invention also provides methods for determining the presence of an ovarian carcinoma in the patient by assaying the concentration of soluble ErbB1 variants in a biological sample from a patient.