X logo officially replaces Twitter's famous bird on mobile app, building headquarters's

X, formerly known as Twitter, has officially retired its famous blue and white bird logo. The icon on the mobile app changed to an "X" late Friday night in the latest phase of a sweeping rebrand the platform's owner Elon Musk announced earlier this month. The company previously introduced the logo on the web and launched the domain X.com, though Twitter.com also remains live. Musk, who acquired the platform for $44 billion late last year, wrote in a post Sunday that the company would soon "bid adieu to the twitter brand and, gradually, all the birds." The transition from Twitter to X reflects Musk's vision to turn the platform into what he has called an "everything app." He shared a video of a large new X logo glowing on top of the company's headquarters early Saturday. City officials in San Francisco opened an investigation into the company Friday for allegedly installing the sign without proper approval, according to the Department of Building Inspection's website. The department filed two active complaints Friday with the descriptions "Structure on roof without permit" and "Unsafe sign" at the address for X's headquarters. In one complaint, a city official said they visited the building and asked company representatives for access to the roof. The official was denied access and told the structure is "a temporary lighted sign for an event." San Francisco's Department of Building Inspection and X did not immediately respond to CNBC's request for comment. The Tesla CEO has long been enamored with the letter "x." SpaceX, Musk's rocket manufacturer, also features an X as its logo, and Musk recently launched a new artificial intelligence startup called xAI, with the lofty goal to "understand the true nature of the universe." But undoing years of branding behind Twitter's blue bird is a move that business analysts consider risky. The company has already struggled to retain advertisers as Musk's changes to the site have led some to fear it's not a safe place for brands to market. Musk recently hired former NBCUniversal advertising executive Linda Yaccarino as CEO, seen as a move to help reassure advertisers. In an email to employees Monday obtained by CNBC's Sara Eisen, Yaccarino wrote that X will develop experiences in video, audio, messaging, banking and payments that will "delight" users. She added that she and Musk plan to work across every team to keep the "entire community up to date." "Time to update.," Yaccarino wrote in a post about the mobile app's logo change Friday.