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Cover  Observations of thin sections passing very obliquely through elements of the endoplasmic reticulum (ER) have allowed researchers to view large surface areas of this intracellular membrane system. As shown in this month's cover art, provided by Eichi Yamada, a striking feature of these sections is the appearance of attached granules that are frequently arranged in consistent patterns, including parallel rows, loops, spirals, circles, and rosettes. These regular arrays represent membrane-bound ribosomes, and in fact, their sizes and shapes are characteristic of the type of mRNA stringing together of ribosomes into membrane-bound polysomes (Christensen, Kahn, and Bourne. [1975]. Am. J. Anat. 178, 1). In a review summarizing his Nobel Lecture (Science. [1975]. 189, 347), George Palade remarked on the distribution of membrane-bound ribosomes by stating that, "Although the endoplasmic reticulum is characterized by high fluidity, the polysomes attached to its cytoplasmic aspect maintain regular characteristic patterns of rather constant geometry. One wonders what prevents them from assuming a random coil conformation; in other words, how does the cell succeed in securing fixed attachment sites on a highly fluid membrane? This riddle must have an interesting answer." It remains to be seen what kind of mechanism underlies this unique distribution of membrane-bound polysomes.Gert Kreibich