Cover picture: Tropomyosin positions revealed in reconstructions of cryoelectron micrographs of muscle thin filaments confirming the steric model of regulation. The positions of tropomyosin strands are shown superimposed on actin (gold) in the absence (left) and presence (center) of Ca²⁺ (tropomyosin strands red and green, respectively). On the right, tropomyosin strands associated with both positions are shown for comparison. Note the movement of tropomyosin toward the inner domains of actin in Ca²⁺ and away from potential myosin binding sites. See the article by Xu et al on p. 985.