Members of the F-box protein family are characterized by an approximately 40-amino-acid F-box domain. SCF complexes, formed by Skp1, Cullin, and F-box proteins, act as protein-ubiquitin ligases. F-box proteins interact with Skp1 through the F-box, and with ubiquitination targets. Therefore, F-box proteins are implicated in cell cycle regulation and signal transduction. We characterized the FBXO45 gene, a recently identified F-box protein, as an estrogen-induced gene. By using bioinformatics, human and mouse FBXO45 (286 aa) showed 100% total-amino-acid identity. Human and mouse FBXO45 genes consisting of 3 exons, and the vicinity of the transcriptional start site had several estrogen receptor binding consensus sequences. Finally, human FBXO45 mRNA was rapidly elevated by 17beta-estradiol in MCF-7 cells.