STAT5A/B represents a therapeutic target for advanced prostate cancer, but the mechanisms underlying increased Stat5 protein levels in prostate tumours are unclear. Now, Haddad et al. have demonstrated amplification at the STAT5A/B gene locus in a significant fraction of clinical prostate cancers, particularly those of high histological grades and in hormone-resistant distant metastases. Functional studies also showed that increased STAT5A/B copy number conferred a growth advantage in prostate cancer cells in vitro and in xenograft tumours in vivo.