The binding of lipophilic hormones, retinoids and vitamins to members of the nuclear-receptor superfamily modifies the DNA-binding and transcriptional properties of these receptors, resulting in the activation or repression of target genes(1,2). Ligand binding induces conformational changes in nuclear receptors and promotes their association with a diverse group of nuclear proteins, including SRC1/p160 (refs 3-5), TIF-2/GRIP-1 (refs 6, 7) and CBP/p300 (refs 4, 5, 8, 9) which function as co-activators of transcription, and RIP-140 (ref. 10), TIF-1 (ref. II) and TRIP-1/SUG-1 (refs 12, 13) whose functions are unclear. Here we report that a short sequence motif LXXLL (where Lis leucine and X is any amino acid) present in RIP-140, SRC-1 and CBP is necessary and sufficient to mediate the binding of these proteins to liganded nuclear receptors. We show that the ability of SRC-1 to bind the oestrogen receptor and enhance its transcriptional activity is dependent upon the integrity of the LXXLL motifs and on key hydrophobic residues in a conserved helix (helix 12) of the oestrogen receptor that are required for its ligand-induced activation function(14). We propose that the LXXLL motif is a signature sequence that facilitates the interaction of different proteins with nuclear receptors, and is thus a defining feature of a new family of nuclear proteins.