It is sometimes the case that a U.S. patent application is filed without all of its required parts. In some cases, it is possible to file certain of the missing parts later without losing the original filing date. This is important in those situations where loss of the original filing date would threaten the patentability of the invention. The last day the missing parts of a U.S. patent application can be filed is the mailing date of the notice plus 2 months. The response period may be extended by up to 5 months by paying extension fees, however, in no case can the period exceed 6 months.