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 recognizes an epitope of a HER2 receptor truncated form, said epitope being defined by a sequence included in SEQ ID NO: 2. The invention also provides a method of cancer diagnosis, 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.