GENEVA (AP) — The Roche Group raised its offer for Illumina Inc. to $51 cash per share Thursday, in a bid to speed up a takeover of the U.S. diagnostics company.

The offer represents an increase of almost 15 percent on Roche’s previous offer of $44.50 per share that was unanimously rejected by Illumina’s board.

“Based on our discussions with Illumina shareholders we have seen interest to accelerate the takeover process,” said Roche CEO Severin Schwan in a statement.

But Illumina immediately cautioned its shareholders to wait before accepting the new offer, which values the San Diego-based DNA sequencing specialist at about $6.5 billion.

“Illumina’s board of directors will thoroughly review Roche’s revised proposal and make a recommendation to stockholders regarding the proposal in due course,” it said. The offer runs until April 20.

Analysts had expected Roche to increase its bid after Illumina’s share price rose to about $50 following the initial takeover offer.

“We see a win-win situation for both sides in a takeover,” Zuercher Kantonalbank said in an analyst note.

Roche shares were down 1.1 percent at 158.10 Swiss francs ($174.93) on the Zurich exchange in morning trading.