Cytosine and adenine (CA) repeats polymorphism (D14S1026) iterating "cytosine and adenine" nucleotide motifs is one of the genomic microsatellites in intron 5 of the estrogen receptor beta (ER beta) gene (14q22-24). Relations between CA repeats polymorphism and several diseases have been shown. Although the relation between number of CA repeats and gene transcription has been actively studied using several genes, results have remained contradictory until this time. In this study, we examined the functional effects of CA repeats polymorphism on transcriptional activity based on our knowledge of the ER beta gene. After preparing four types of reporter gene constructs containing 15, 18, 24 or 27 CA repeats, luciferase reporter gene assays were performed. Relative luciferase activities of these constructs were not significantly different from that of the no inserted vector and variation of CA repeats did not affect these activities. Our results indicate that CA repeats polymorphism might not only affect transcriptional activity but also other processes of gene expressions. Further studies are needed to clarify the specific functions of CA repeats polymorphism in the ER beta gene.