Abstract: The present invention relates to a method of diagnosis and therapy of cancers expressing the HER2 receptor. The invention provides antibodies or fragments thereof that recognise an epitope of the HER2 receptor truncated form CTF-611, said epitope being defined by a sequence included in SEQ ID NO: 2, and that are capable of discriminating between CTF-611 and CTF-616 (represented by SEQ ID NO:7), preferably additionally capable of discriminating between CTF-611 and CTF-613 (represented by SEQ ID NO:6). The invention also provides a method of cancer diagnosis using the disclosed antibodies, which comprises the detection of the presence of the HER2 receptor truncated form consisting of the amino acid sequence SEQ ID NO: 1 in a patient sample.