Slack is set to be Silicon Valley’s next big debut on the stock market, with plans to go public on Thursday. The start of public trading, through a rare direct listing in which no new shares are sold, will test investors’ confidence in the messaging firm’s growth potential. Already, there are signs that the company will be valued at a much higher level on the public markets than in its fundraising rounds as a private firm.

But the fact that Slack is planning a direct listing—in which all shareholders are free to sell immediately—raises the prospect of a sell-off, especially by venture capital investors who typically don’t hold public shares long term. Trading could be particularly influenced by four investors—venture capital firms Accel, Andreessen Horowitz, Social Capital and Japanese conglomerate SoftBank—which together hold more than half of Slack’s outstanding shares.