Cover image

Cover The evolutionary origin of uneven distribution of gene family sizes. Data presented in this issue suggest that two major factors contribute to uneven distribution of gene family sizes: innate differences in protein folds (manifested by differences in their "designabilities") and their evolutionary history. On the cover, the more designable protein (red) is schematically illustrated as being contained in a cylinder made from more sequences than the blue structure. More designable folds are characterized by higher contact density. As the illustration suggests, many more sequences exist that can be stable in highly designable folds. Original folds, belonging to the last universal common ancestor, are, on average, more designable than all presently existing folds. (For details, see Shakhnovich et al., pp. 385–392.)