Biotechnology giant CSL will pay $US450 million ($A655.9 million) for a gene therapy candidate for haemophilia B, which may help people with the blood clotting disorder stop bleeding.

US company uniQure produces the program called etranacogene dezaparvovec, or AMT-061, which could be one of the first gene therapies to provide long term benefits to people with haemophilia B.

While the program is in phase three trials, it promises to increase Factor IX plasma levels in the blood so that recipients have a greatly reduced tendency to bleed.

Patients would only need one treatment, which would remove the need for ongoing therapies.

CSL will have the exclusive global rights to commercialise the program.

It will also pay royalties and sales milestone fees to uniQure.

UniQure will complete the phase three trial and boost manufacturing in order to supply CSL.