26 C.F.R. § 53.4943-1
Generally, under section 4943, the combined holdings of a private foundation and all disqualified persons (as defined in section 4946(a)) in any corporation conducting a business which is not substantially related (aside from the need of the foundation for income or funds or the use it makes of the profits derived) to the exempt purposes of the foundation are limited to 20 percent of the voting stock in such corporation. In addition, the combined holdings of a private foundation and all disqualified persons in any unincorporated business (other than a sole proprietorship) which is not substantially related (aside from the need of the foundation for income or funds or the use it makes of the profits derived) to the exempt purposes of such foundation are limited to 20 percent of the beneficial or profits interest in such business. In the case of a sole proprietorship which is not substantially related (within the meaning of the preceding sentence), section 4943 provides that a private foundation shall have no permitted holdings. These general provisions are subject to a number of exceptions and special provisions which will be described in following sections.
26 C.F.R. §53.4943-1