The LIM domain is a cysteine-rich domain composed of 2 special zinc fingers that are joined by a 2-amino acid spacer. Some proteins are constituted by LIM domains only while others contain a variety of different functional domains. LIM proteins form a diverse group which includes transcription factors and cytoskeletal proteins. The primary role of LIM domains appears to be in protein-protein interaction, through the formation of dimers with identical or different LIM domains or by binding distinct proteins. In LIM homeodomain proteins, LIM domains seem to function as negative regulatory domains. LIM homeodomain proteins are involved in the control of cell lineage determination and the regulation of differentiation, and LIM-only proteins may have similar roles. LIM-only proteins are also implicated in the control of cell proliferation since several genes encoding such proteins are associated with oncogenic chromosome translocations. In analyzing sequence relationships between LIM domains we suggest that they may be arranged into 5 groups which appear to correlate with the structural and functional properties of the proteins containing these domains.