Springtime finds hopeful anglers baiting hungry fish with twitching worms, both live and artificial. Fish prefer the large annelids, but Kemp and coworkers have knotted on their lines the small, alluring nematode Caenorhabditis elegans, which twitches spasmodically when the aptly named protein twitchin goes missing from its muscle cells. And they've caught a big one! On page 636 of this issue, these authors report that the giant protein kinase twitchin, which has a relative molecular mass of 750K and is found in nematode muscle cells, and the protein S100A1(2), a member of the S100 family of calcium-binding proteins, make up a third new calcium-regulated system in muscle which may be of great importance in organizing muscle structure and maintaining its resting tension. They show that a fragment of twitchin containing the autoinhibited kinase domain is specifically activated in a calcium-dependent and zinc-enhanced manner by S100A1(2), but not by the S100B(2) isoform with which it shares 60 percent homology....