Does a constructive amendment to a grand jury indictment constitute a per se violation of the grand jury clause?

Id., 858 F.2d at 98 (quoting Gaither v. United States, 413 F.2d 1061, 1071 (D.C.Cir.1969)). There is, however, one significant difference between the two--a constructive amendment of the indictment is considered to be a per se violation of the grand jury clause, while a defendant must show prejudice in order to prevail on a variance claim. See id. We need not decide here whether a difference between an overt act proved at trial and that alleged in the indictment could ever be so significant as to give rise to a valid constructive amendment claim. It is enough for us to say (1) that it was not improper per se for the district court to instruct the jury that the overt act element could be satisfied by an act not listed in the indictment; and (2) that, under the facts presented in this case, the appellants have established neither that the district court constructively amended the indictment nor that there was a prejudicial variance.