Some of the recently identified coactivators which interact with members of nuclear hormone receptors contain a stretch of homopolymeric glutamines (poly-Q). Length of poly-Q in several genes are known to be polymorphic in healthy subjects, and extraordinary expansion of poly-Q in specific genes is known to cause neurodegenerative disorders. In the present study, we investigated whether such polymorphism can be observed in two coactivators, CBP (CREB [cyclic AMP responsive element binding protein]-binding protein) and AIB1/ACTR (amplified in breast cancer-1/ACTR, also called RAC3/TRAM-1). The genomic regions encoding the poly-Q were amplified by means of PCR using fluorescence labeled primer and analyzed by an automatic sequencer. While contiguous glutamine residues inAIB1/ACTR ranged from 26 to 32 with a heterozygosity of 54%, no polymorphism could be observed in poly-Q of CBP among 54 unrelated subjects. These results suggest that the residue in CBP may play a critical role in the function so that individuals with CBP containing different sizes of poly-Q might have been eliminated. It has been reported that AIB1/ACTR is overexpressed in some of the cell lines derived from breast cancer. If the length of poly-Q alters the stability of AIB1/ACTR and/or potency to enhance hormone action through nuclear receptors, the length of poly-Q is likely to be one of the genetic factors affecting not only susceptibility to breast cancers but also the sensitivity to hormones. This polymorphism should also be tested in patients with neurodegenerative disorders of unknown cause.