Genentech of South San Francisco is one of the most profitable biotech companies in the business.
Swiss drugmaker Roche on Monday officially launched its $42 billion hostile offer for the 44 percent of Genentech Inc. it doesn't already own.
Roche already holds a 56 percent stake in Genentech, which is based in South San Francisco, Calif.
Roche's latest offer, $2.50 per share less than an offer rejected by Genentech directors in July, has sparked concern from analysts that key personnel at the biotech pioneer could leave, depending on any potential deal.
Roche estimated the deal value at $42 billion in a regulatory filing Monday.
Roche said its new offer is aimed directly at shareholders, bypassing the board.
The offer is significantly lower than the $100-per-share value analysts had been touting for several months.
"While we have changed our approach to the transaction, our plan on how we will combine the two companies remains unchanged," said Roche Chairman Franz Humer.
The company's offer expires at midnight March 12.
Roche said it plans to complete the acquisition if it owns 90 percent of Genentech shares at that time.
Shares of Genentech fell 30 cents to $82.70.