System and method to inhibit heregulin activated processes and other methods using soluble ErbB3 and method to produce soluble ErbB3

An isolated nucleic acid selected from the group consisting of:

• a) a nucleic acid which encodes a protein comprising the amino acid sequence SEQ ID NO. 1;
• b) a nucleic acid which encodes a protein comprising an amino acid sequence which is at least 90% identical to SEQ ID NO. 1 and which has at least 50% of the biological activity of the protein SEQ ID NO. 1;
• and
• c) a nucleic acid which is complementary to nucleic acid a) or b).

An assay for determining the concentration of ErbB3 in a biological sample from a mammal, the assay comprising:

• a) obtaining a biological sample from the mammal;
• b) contacting an amount of a first purified antibody that specifically reacts with a first epitope of the extracellular ligand binding domain of ErbB3 with the mammal biological sample to be tested, wherein the first purified antibody is modified with a first labeling moiety;
• c) contacting the sample with an amount of a second purified antibody that specifically reacts with a second epitope of the extracellular ligand binding domain of ErbB3, wherein the second purified antibody is modified with a second labeling moiety, and wherein the second purified antibody does not competitively inhibit the binding of the first purified antibody;
• and
• d) detecting the co-presence of the first and second labels to determine the concentration of the soluble epidermal growth factor receptor complexed with the antibodies;
• wherein the antibodies which competitively inhibit the binding of ErbB3.