Shares in Alphabet sank after a report that ChatGPT owner OpenAI is developing a web search product that would compete with Google.

OpenAI’s service would be partly powered by Microsoft’s Bing search engine, The Information reported (paywall), citing an unidentified person familiar with the matter. Alphabet fell as much as 3.8% on Thursday, far underperforming the Nasdaq 100, which dipped 0.3%.

Alphabet has been dogged by concerns about risks posed by rival artificial intelligence services to its Google search business, which generates the majority of the company’s revenue through the sale of digital advertising. Microsoft, which has invested billions of dollars in OpenAI and incorporated ChatGPT features into Bing last year, has so far failed to make inroads in market share.

Baird analyst Colin Sebastian said the risk to Alphabet from an OpenAI search product is “modest” and noted that it won’t be easy for the start-up to create a viable competitor.

Read: Google opens access to Gemini in race to beat OpenAI

“Search is extremely challenging to get right — and Google has fended off many competitors over the years with the quality of search results,” he wrote in a research note. “Alternative search engines probably need to be better than Google to really shift user behaviour, which could be a challenge even for OpenAI.”