17 CFR §9.23

1. (a)Time for filing answering brief. Within thirty days after service of the appeal brief, the exchange must file with the Commission an answering brief.
2. (b)Contents of answering brief. The answering brief generally must follow the same style as prescribed for the appeal brief but may omit a statement of the issues or of the case if the exchange does not dispute the issues or the statement of the case contained in the appeal brief.
3. (c)Length of answering brief. Without prior leave of the Commission, the answering brief may not exceed thirty-five pages, exclusive of any table of contents, table of cases, index and appendix containing transcripts of testimony, exhibits, statutes, rules, regulations or similar materials.