Roche, of Basel, said it plans on paying $44.50 per share for Illumina, an 18.1% premium to Tuesday's close and a 64% premium to the stock's Dec. 21 close, the day before Roche's interest became public.

"It is our strong preference to enter into a negotiated transaction with Illumina, and we remain willing to engage in a constructive dialogue with Illumina to jointly develop an optimal strategy for maximizing the value of our combined business," Roche CEO Severin Schwan said in statement.

Illumina's technology allows doctors to identify the genetic make-up of tumors and craft targeted treatments. Roche is very interested in expanding its analytical capabilities to support drug sales and last summer signed a cooperation agreement with Hamburg's Evotec AG to use Evotec's genetic analysis tools for its own cancer-battling medications.

Roche said it could finance the acquisition with existing cash and credit lines but said it would move its applied sciences activities to Illumina's San Diego headquarters.

The Swiss company has successfully used hostile approaches in the past to reel in rivals. In 2008 it won over shareholders of Tucson, Ariz., tissue-analysis company Ventana Medical Systems Inc. by sweetening an unwanted $3 billion bid to $3.4 billion.

That same year it convinced minority shareholders of San Francisco's Genentech Inc. to hand over a 44% stake it didn't already own after eight months of haggling - it also increased its approach to $46.8 billion from an initial $43.7 billion.

Illumina's board Wednesday acknowledged the offer.

"Consistent with its fiduciary duties and responsibilities, and in consultation with its financial and legal advisers, Illumina's board of directors will thoroughly review Roche's proposal and make a recommendation to stockholders in due course," the company said.

Greenhill & Co. and Citigroup Inc. are providing financial advice to Roche, while Davis Polk & Wardwell LLP act as counsel.

Goldman, Sachs & Co. and Bank of America Merrill Lynch are Illumina's financial advisers, with Dewey & LeBoeuf LLP handling legal details.