Abstract: Neoplastic and other diseases can be diagnosed by assaying a human test sample e.g. body fluid, tissue or cultured tumor explant cells, for structurally altered or abnormally expressed growth factor receptors or for the RNA transcripts of genes which encode them. For example, the assay can be for truncated EGF receptor having at least a portion of its mature amino terminus deleted. Antibodies, capable of binding a predetermined amino acid sequence within the EGF receptor, are also useful in diagnosis and therapy as are conjugates of an immunogenic polymer bound to a polypeptide fragment of EGF receptor. DNA and RNA encoding EGF receptor or fragments thereof are also described.