[joshjkim]

As a follow-up: here's sample language from a founder stock purchase agreement (from orrick's start-up forms - https://www.orrick.com/Events-and-Publications/Documents/197...), where you can see it literally references IPO, underwriter's, securities laws, etc.

Lock-Up Agreement. If so requested by the Company or the underwriters in connection with the initial public offering of the Company’s securities registered under the Securities Act of 1933, as amended, Purchaser shall not sell, make any short sale of, loan, grant any option for the purchase of, or otherwise dispose of any securities of the Company however or whenever acquired (except for those being registered) without the prior written consent of the Company or such underwriters, as the case may be, for 180 days from the effective date of the registration statement, plus such additional period, to the extent required by FINRA rules, up to a maximum of 216 days from the effective date of the registration statement, and Purchaser shall execute an agreement reflecting the foregoing as may be requested by the underwriters at the time of such offering.