17 CFR §200.41

A quorum of the Commission shall consist of three members; provided, however, that if the number of Commissioners in office is less than three, a quorum shall consist of the number of members in office; and provided further that on any matter of business as to which the number of members in office, minus the number of members who either have disqualified themselves from consideration of such matter pursuant to § 200.60 or are otherwise disqualified from such consideration, is two, two members shall constitute a quorum for purposes of such matter.