Tscan Therapeutics is trading at $0.21, down 5.58% after-hours compared with the regular-session close; no clear company-specific catalyst was identified for the October 20 move. - The company’s press-release page lists no October 20 announcement; its latest listed release is dated September 2. - A shareholder-meeting filing proposing a reverse split predates the move and does not establish its cause. - Broader U.S. indices finished higher, so the supplied market context does not explain the decline.