Well, the gap -- of having no present supervisor's letter in your folder -- will make them wonder what's going on. Why give them a chance to wonder about your candidacy? So fill in the gap by taking an initiative to honestly and credibly explain why. Keep your explanation brief. In doing so, do NOT bad-mouth your supervisor, show your gratitude to him, but be positive and talk about newer/fresher yet specific academic/intellectual challenges you will encounter in the US . . . challenges that are not quite possible in Korea/Japan for you. This is the best you can do, under the circumstances, and hope for the best. oohi ashu