SAN FRANCISCO — Arent Fox is preparing to launch an office in San Francisco on May 1, joining a number of firms that plan to open up shop in the city.

The firm will open the office with at least five lawyers who will be based in San Francisco full-time. Four of those lawyers will be transferred from the firm’s other offices, and one has been hired from outside the firm, said partner Robert O’Brien, who will lead the office. O’Brien, who also manages the firm’s office in Los Angeles, will divide his time between Northern and Southern California along with partner Stephen Larson.