Gilead Sciences Inc. (Nasdaq: GILD) will buy Kite Pharma Inc. (Nasdaq: KITE) for about $11.9, making one of its biggest-ever deals for a breakthrough new cancer treatment and helping it diversify away from its eroding franchise of medicines for hepatitis C infections.

Strategic buyers after cancer treatment companies. In 2016, Bristol-Myers Squibb Co. (NYSE: BMY) acquired Cormorant Pharmaceuticals; Pfizer Inc. (NYSE: PFE) bought Medivation; and Jazz Pharmaceuticals plc purchased Celator Pharmaceuticals.

With Kite, Gilead will gain a foothold in one of the most promising fields in oncology: treatments known as CAR-T that re-engineer the body’s own immune system to fight tumors. The drugmaker said that it will pay $180 a share in an all-cash deal.

“This is a pivot to cellular therapy as our main strategy going forward,” Gilead CEO John Milligan said on a call with analysts. The company, which hasn’t had great success in cancer so far, will continue to look for assets. “We’re not going quiet after this deal.”

The acquisition caps more than a year of search as valuations for biotechnology companies focused on breakthrough therapies soared and the best got bought by rivals. Gilead, seeking to fill a gap left by declining sales of hepatitis C medicines, said last in 2016 it was feeling “ an urgency to look at external opportunities.”

Gilead was ready to pay the price for Kite, whose shares have tripled in 2017. Among Kite’s most-anticipated new treatments is a drug for non-Hodgkin lymphoma, which is awaiting for U.S. approval. Kite said it could be ready for a launch as soon as September, sending the shares new records. Kite is racing Swiss giant Novartis AG to get its first product on the market.

The Kite acquisition also signals an evolution in Gilead’s thinking about CAR-T as pressure from investors grew to make a deal and some biotechs, including Kite, showed very effective results in trials. As recently as in 2016, CEO Milligan expressed some skepticism about the therapies, saying they’re complex and labor-intensive — more like a bone marrow transplant than a drug.

Because they harness the body’s own immune system, CAR-Ts have had dramatic results in trials for some patients, but also showed strong and sometimes lethal side effects with others. CAR-T are also complex to manufacture. Unlike pills or chemotherapy, the treatments can’t be stockpiled. They’re living drugs, and the production process entails drawing a patient’s blood; extracting the T-cells; inserting a gene that will enable them to identify tumors as targets; then infusing the cancer-killing compound back into the patient at a specialized medical center. But Gilead watched Kite becoming more and more attractive through the summer, Milligan said.

“It became clear that the side effects would be more manageable and more importantly that the manufacturing would work,’’ Milligan said. “They’ve solved the manufacturing problem. It all added up to this being a very unique, special opportunity for us, and this became very compelling.”

The transaction, scheduled to close in the fourth quarter, would be Gilead’s largest by equity value, surpassing the 2012 acquisition of Pharmasset for $11.2 billion. The Kite agreement was unanimously approved by the two boards and will start add to earnings three years after completion.

BofA Merrill Lynch and Lazard advised Gilead, while Centerview Partners provided advice to Kite, along with Jefferies LLC and Cowen & Co. For legal counsel, Gilead was advised by Skadden, Arps, Slate, Meagher & Flom served as legal counsel to Gilead, while Sullivan & Cromwell LLP and Cooley LLP counseled Kite.

Additional reporting by Mergers & Acquisitions’ Demitri Diakantonis