Under Internal Revenue Code Section 1045, a type of stock for startup companies that allows you, under certain conditions, to sell QSB stock at a 0% capital gains rate or to defer paying any tax on the gain if you roll over the proceeds into another startup within 60 days from the date of sale. There are detailed rules that determine whether your stock and the new company's stock is QSB stock, and how to handle the rollover to the new company. See a related article on this website.