Pfizer Inc said on Wednesday it has acquired a 15 percent stake in Vivet Therapeutics, and has an exclusive option to fully acquire the privately held French company that develops gene therapies for liver disorders.

Under the terms of the deal, Pfizer said it has paid about $51 million upfront and may pay up to $635.8 million in clinical and regulatory milestone payments, inclusive of the payment to exercise the option.

The partnership comes at a time when gene therapy is gaining momentum, and big drugmakers including Roche AG have begun pushing into the space.

In a joint statement, the companies said Pfizer can exercise its option to acquire all of Vivet following its delivery of certain data from an early-stage clinical trial for its VTX-801, an experimental treatment for Wilson disease.

(Reporting by Manojna Maddipatla in Bengaluru;' Editing by Shinjini Ganguli) ((manojna.kalyani@thomsonreuters.com; within the U.S. +1 646 223 8780, outside the U.S. +91 80 6749 1692; Reuters Messaging: manojna.kalyani.thomsonreuters.com@reuters.net))