The U.S. Federal Trade Commission filed a complaint to block Illumina Inc.'s planned $1.2 billion acquisition of Pacific Biosciences of California Inc.

The FTC expressed concerns that Illumina aims to "unlawfully maintain its monopoly" in the U.S. for DNA sequencing — used in genetic research and clinical testing — by buying Menlo Park, Calif.-based company Pacific Biosciences.

According to the complaint, PacBio in recent years has made advancements which make it a closer alternative to San Diego-based Illumina. Customers have switched some business to PacBio from Illumina, and the former is poised to take more business from the latter in the future, the FTC said.

Further, since the companies drive each other's innovation, the commission believes the deal will reduce efforts to innovate and develop new products.

The agency voted 5-0 to issue the administrative complaint and to authorize staff to seek a temporary restraining order and preliminary injunction, if needed, to prevent the transaction from moving forward. A trial is set to begin Aug. 18, 2020.

In September, Illumina extended the closing date of the acquisition to Dec. 31.

Illumina's stock dropped 1.31% to $323.45 at market close on Dec. 17, while Pacific Biosciences' stock fell 7.85% to $4.93 in the after-hours market.