Cover picture: Measurement and normalization of fluorescence resonance energy transfer (FRET) in cells. The figure shows a cell expressing donor and acceptor proteins at a ratio of 2:1 (arrowheads) and a cell with a 4:1 donor to acceptor ratio (arrows). Image analysis shows the relative degree of net FRET (D) and normalized FRET (E) within the cells. See the article by Xia and Liu on p. 2395.