Washington-based law firm Arnold & Porter is absorbing mid-size San Francisco firm Howard Rice Nemerovski Canady Falk & Rabkin, leaders at both firms said Tuesday.

The merger, effective Jan. 1, is to bring Arnold & Porter’s worldwide attorney headcount to more than 800, including 80 from Howard Rice, perhaps best known for representing the Winklevoss twins in litigation with Facebook founder Mark Zuckerberg.

The move includes layoffs of a combined 27 staff from both firms. The majority of the layoffs are to be at Howard Rice, said Arnold & Porter Managing Partner Richard Alexander. No lawyers are to be let go, Alexander said.

Arnold & Porter is to gain two new practice groups through the merger: Howard Rice’s attorney liability practice, which represents law firms and lawyers in malpractice and ethics issues, and a standalone labor and employment practice. It also doubles the firm’s California presence from about 80 to 160 lawyers in San Francisco, Silicon Valley and Los Angeles.

“We view the West Coast as an important part of our national and international strategy,” Alexander said. “They’re a highly talented law firm, so the talent proposition here was fantastic.”

Howard Rice Chairman Doug Winthrop is to be co-head of Arnold & Porter’s San Francisco office with current Bay Area Chairman Trenton Norris, and is to join the firm’s policy and compensation committees.

“Although in the past we have preferred to remain independent, we believe this time the opportunity for our clients and our attorneys was too good to pass up,” Winthrop said in a statement.