 The main concern of a long term treatment with HU is its potential leukaemogenicity: based on the mechanism of action, HU can potentially accelerate the accumulation of mutations in DNA and increase the risk of leukaemic transformation Dingli et Tefferi, 2006. However, there is currently no clear clinical data to confirm leukaemogenicity of HU in patients with PV Tefferi, 2012. 