Under the deal, Genzyme will pay the $1 billion on June 22 and receive about 15.5 million shares, currently estimated to be approximately 80 percent of the shares to be repurchased. The total number of shares ultimately repurchased under the deal will be determined upon final settlement.

The company said proceeds from a recently concluded debt offering will be used to support the buyback, which is the first part of a broader $2 billion buyback program announced in May.

Genzyme shares rose 87 cents to $52.30 in premarket trading.