French pharmaceutical giant Sanofi said on Monday it had agreed to buy US biotech firm Synthorx to boost its immuno-oncology portfolio.

Sanofi said it will pay $2.5 billion for the US company which specialises in treatments prolonging and improving the lives of cancer patients or those with auto-immune disorders.

Immuno-oncology therapies aim to mobilise the body's immune system to fight cancer cells.

California-based Synthorx has developed a way to expand the human genetic code by adding a new DNA base pair in what it says is a "first-of-its-kind" approach boosting the chances of cancer therapies to be effective.

The French company is paying $68 in cash for Synthorx, valuing the target company's stock at 172 percent above its closing price on Friday.

The acquisition is "aligned with our goal to build our oncology franchise with potentially practice-changing medicines and novel combinations," Sanofi CEO Paul Hudson said in a statement.

Sanofi is present in 100 countries and employs more than 100,000 people.