Roche Holding, the largest Swiss drug maker, offered to buy the roughly 44% stake of U.S. biotechnology company Genentech Inc., for $43.7 billion to gain the largest U.S. maker of cancer medicines. The Basel Company already owns 56 percent of Genentech and markets many of its treatments.

Investors in Genentech, which is based in South San Francisco, California, would get $89 a share, 8.8 percent more than the July 18 closing price, Roche declared on Monday in an e-mailed statement.

“We will be better able to share technologies and expertise in pharmaceuticals and diagnostics across the group and broaden the mutual access to the external innovation networks of both companies,” said chief executive Severin Schwan, 41, the former head of the Swiss company’s diagnostics unit, in a statement. “The transaction will also unlock synergies by leveraging the scale of the combined operations in the U.S. and improving operational efficiency.”

Roche rose 1.6 percent to 179.60 Swiss francs on July 18 in Zurich trading. The shares have declined 8.2 percent this year, outperforming the Bloomberg Europe Pharmaceutical Index of 19 companies, which has declined 12 percent. Genentech rose 0.8 percent to $81.82 July 18, and has gained 22 percent this year.

On July 14, Genentech said second-quarter profit rose 4.7 percent and revenue increased 8 percent to $3.2 billion, led by U.S. sales of Avastin. The medicine, first approved in 2004 for colon cancer and for lung malignancies two years later, is being studied against 20 tumor types worldwide.