The present invention relates to a method for the diagnosis or prognosis of breast cancer metastasis that comprises determining whether the MAF gene is amplified in a primary-tumour sample. Furthermore, the invention also relates to a method for the diagnosis or prognosis of ER breast cancer mestastasis and also a method for determining the propensity for developing metastasis to bone as opposed to metastasis to other organs, comprising determining the expression level of the c-MAF gene. Lastly, the invention relates to the use of a c-MAF inhibitor as therapeutic target for the treatment of ER breast cancer metastasis.
