The LIM domain is a specialized double-zinc finger motif found in a variety of proteins, in association with domains of divergent functions or forming proteins composed primarily of LIM domains. LIM domains interact specifically with other LIM domains and with many different protein domains. LIM domains are thought to function as protein interaction modules, mediating specific contacts between members of functional complexes and modulating the activity of some of the constituent proteins. Nucleic acid binding by LIM domains, while suggested by structural considerations, remains an unproven possibility. LIM-domain proteins can be nuclear, cytoplasmic, or can shuttle between compartments. Several important LIM proteins are associated with the cytoskeleton, having a role in adhesion-plaque and actin-microfilament organization. Among nuclear LIM proteins, the LIM homeodomain proteins form a major subfamily with important functions in cell lineage determination and pattern formation during animal development.