“…Although the existing mathematical methods, such as direct methods (Ladd & Palmer, 1980;Schenk, 1991;Woolfson & Fan, 1995), multiwavelength anomalous scattering (MAD) (Hendrickson, 1991), and many others (Rossmann, 1972;Bricogne & Gilmore, 1990) are useful phasing techniques for small and macromolecular crystals, the phase problem is not yet completely solved because direct methods run into dif®culties with macromolecules since the probability of having the correct phase relation is inversely related to the number of atoms in the crystal unit cell (Ladd & Palmer, 1980) and suitable wavelengths and heavy-atom derivatives may not always be obtainable. Recently, multiple diffraction techniques utilizing interference effects (Post, 1977;Chapman et al, 1981;Chang, 1982;Juretschke, 1982;Hoier & Aanestad, 1981;Hu È mmer & Billy, 1986;Chang, 1987, and references therein; Mo et al, 1988;Shen & Finkelstein, 1990) have demonstrated their capability of physically measuring re¯ection phases qualitatively and quantitatively (Chang & Tang, 1988;Weckert & Hu È mmer, 1990) without invoking multiwavelength experiments and heavy-atom derivatives for macromolecular crystals (Hu È mmer et al, 1991;Chang et al, 1991;Weckert & Hu È mmer, 1997, and references therein). Unfortunately, the intensity pro®les of multiple diffraction used in phase determination are usually obtained one at a time.…”