When Celgene announced Monday that it will acquire Seattle-based biotech Juno Therapeutics for $9 billion, the development was a surprise to many — including some inside Juno itself, according to an email CEO Hans Bishop sent employees.

But the New Jersey biotech giant had several good reasons to want to buy Juno, which it outlines in a presentation filed Monday with the SEC.

One of the main motivators for the sale was JCAR017, Juno’s self-described flagship treatment that is being studied in non-Hodgkin lymphoma. It’s one of several CAR T immunotherapies Juno is developing, treatments that genetically reprogram patients’ immune cells to fight cancer.

When Juno was forced to pull its most advanced treatment in March following patient deaths during clinical trials, it made a massive investment in speeding up the progress of JCAR017 — and it seems to be paying off, with the treatment finding clinical success.

In the presentation to investors, Celgene CEO Mark Alles said JCAR017 could be a “best-in-class profile” that would provide an anchor therapy in treating non-Hodgkin lymphoma, particularly interesting to Celgene given its history in developing blood cancer treatments.

The presentation says Celgene sees an opportunity to become a global leader in treating non-Hodgkin lymphoma, and that JCAR017 could generate as much as $3 billion in sales at its peak. It also says Celgene expects Juno to begin adding to Celgene’s revenue by 2020, and points to Juno’s cell manufacturing platform as an important asset.

But JCAR017 and other scientific advances weren’t the only motivators — Celgene also says that changes in the U.S. tax code were a key development that drove the deal. “US tax reform enables improved visibility into after-tax cash flows and access to ex-US cash to fund transaction,” the presentation says. The recent changes to the federal tax code have been well-received by the biotech and life science industry.

The deal has many in Seattle’s tech community worried that it may be about to lose yet another biotech powerhouse. The city has a strong history in medical research and biotechnology but has struggled to keep large public companies like Juno around.

For its part, Juno doesn’t seem to be interested in moving anywhere. The company has turned down opportunities to relocate in the past said it will remain in its Seattle area locations, including its brand new, custom-built headquarters in Seattle’s South Lake Union neighborhood. Celgene also has an office in Seattle.

“Seattle will be the world-wide center of excellence for cell therapy and immune-oncology within Celgene, and I am confident we will be empowered to do more,” Bishop, Juno’s CEO, said in an email to employees. “The driving force behind Juno has always been our focus on delivering for patients, it’s the value that I know is closest to all our hearts. We must nurture and keep that focus. Patients urgently need us to deliver new therapies.”