French drugmaker Sanofi-Aventis completed its improved $20.1 billion offer for Genzyme allowing it to begin merging the U.S. biotech into its business and add rare diseases to its growth areas.

Sanofi said 84.6% of Genzyme's common stock had been tendered, with shareholders receiving $74 in cash and one certificate per share entitling them to future payments if specified milestones tied to several Genzyme treatments are met.

The French company also said on Monday it would give remaining Genzyme shareholders a further four days until Thursday to tender their shares.

"Through the acquisition, Genzyme will become an important new platform in Sanofi-Aventis' sustainable growth strategy and expand the company's presence in biotechnology," Sanofi said in a statement.

"Sanofi-Aventis is making Genzyme its global center for excellence in rare diseases."

Chief Executive Chris Viehbacher added in the statement that the integration process for the two companies was "progressing well and remains on track".

Sanofi clinched its long-sought deal to buy Genzyme in February with a sweetened offer that Genzyme's board unanimously recommended to its shareholders.

The certificates, or contingent value rights (CVR), issued as part of the transaction will begin trading on the U.S. Nasdaq stock exchange later on Monday under the ticker.

The CVR runs through 2020 and if all milestones are achieved could pay $3.8 billion in total.

Genzyme investors can be paid $1 per CVR if Genzyme meets specified production levels of Cerezyme and Fabrazyme this year after the two key rare disease drugs were in short supply due to contamination problems at a manufacturing plant.

Another dollar will be paid if the U.S. health regulator, the Food and Drug Administration (FDA), clears Genzyme's Lemtrada multiple sclerosis drug for marketing, which could come in 2012.

Further away are milestone fees tied to the sales performance of the drug whose sales potential was a key bone of contention between the two companies during the takeover battle.

The face value of one CVR is as much as $14, to be paid over time if targets are met, but analysts expect it will be highly discounted in the market given the risks tied to a drug that has yet to be approved and the long timeframe.