For example, although both STAT5a and STAT5b are constitutively activated in numerous human cancers, including human leukaemias harbouring the Philadelphia chromosome3 which leads to expression of the Bcr-Abl fusion protein, the inhibition of STAT5b was shown to reduce tumour cell proliferation more than the inhibition of STAT5a did4, 5. Here, STAT5B is linked to cancer.