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Three modules must be enabled to run the migrations: your site’s migration module, migrate_d2d, and the migrate module. You can either enable these using Drush or through the GUI. You may also enable migrate_ui if you wish to do the migrations through the GUI (however, it does work better using Drush, especially if you are migrating from multiple databases to a single destination site. I will touch on multiple database migrations in the section titled Migrating from Multiple Sources to a Single Destination).