17 C.F.R. § 9.26
Within twenty days after receipt of the answering brief, the Division of Market Oversight, Market Participants Division, and the Division of Clearing and Risk may file with the Proceedings Clerk a notice of intention to participate in the proceedings as amicus curiae. Within thirty days after filing the notice of intention to participate, the Division may file a brief as amicus curiae. Without prior leave of the Commission, the brief may not exceed thirty-five pages. The brief must be filed and served on the appellant, exchange and any other parties to the proceeding in the manner specified by these rules. Within ten days after service of the Division's brief, any party may file a reply to the Division's brief. After the filing of the notice of intent to participate, no employee of the Division(s) filing the notice may thereafter make any communication relating to the proceeding, other than on the record of the proceeding before the Commission, to any Commissioner or Commission decisional employee.
17 C.F.R. §9.26