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Twitter shares drop after co-founder Evan Williams announces big stock sale

Twitter's co-founder and current chief executive of Medium, Evan Williams, announced Thursday he is selling shares in the social media company he helped start. Twitter shares dropped more than 1 percent following the news.

"It actually pains me to be selling at this point, but this sale is all about personal context, not company context," Williams wrote in a Medium post.

Williams explained that the majority of his current assets are in the form of Twitter stock, which he plans to begin to liquidate through a Securities and Exchange Commission rule 10b5–1 into 2018.

Williams already sold about $4 million worth of Twitter stock Wednesday, according to filings with the SEC. But a spokesperson confirmed to CNBC that Williams plans to sell no more than 30 percent of his shares, thereby reducing his stake in the company from about 5 percent to slightly under 4 percent.

"I'm proud to be on the board and optimistic about the future of [Twitter]," Williams wrote.

The co-founder was a regular seller of Twitter stock between 2014 and 2015 — a report from InsiderScore shows — before he took a break from his 10b5-1 plan throughout the majority of 2016.

"The similarity of [his latest] selling suggests Williams is embarking on another round of regular patterned sales," InsiderScore wrote in a brief to clients on Thursday.

Williams served as Twitter's chief executive officer for a brief time, before stepping down in 2010. He's now heading up Medium, a blogging platform.

Twitter's stock has struggled lately and is down more than 16 percent over the past 12 months, as Chief Executive Jack Dorsey tries to turn things around. Dorsey also co-founded the company with Williams.