Cyclins control cell-cycle progression by regulating the activity of cyclin-dependent kinases. Cyclin I was recently added to the cyclin family of proteins because of the presence of a cyclin box motif in the deduced amino-acid sequence. Cyclin I may share functional roles with cyclin G1 and G2 because of the high structural similarity between their deduced amino-acid sequences. However, the biological and functional roles of this subclass of cyclins remain obscure. The mouse cyclin G1 and G2 genes have previously been cloned and characterized. In this report, we describe the cloning of the mouse homolog of cyclin I. The cyclin I cDNA sequence was used to determine the genomic organization of the mouse cyclin I gene which co-localizes with cyclin G2 to chromosome 5E3.3–F1.3. Cyclin I was transcribed from seven exons distributed over more than 19kb of genomic sequence. The expression of cyclin I was determined in various tissues, but no clear correlation with the proliferative state was found. Furthermore, in contrast to cyclin G1, cyclin I expression was stable during cell-cycle progression after partial hepatectomy in both the absence and presence of DNA damage. Transient expression of cyclin I–green fluorescent protein (GFP) fusion proteins in cell lines showed that cyclin I was distributed throughout the cell in contrast with the mainly cytoplasmic localization of cyclin G2 and nuclear localization of cyclin G1. Our results indicate that despite the close structural similarity between cyclin G1, G2 and I, these three proteins are likely to have distinct biological roles.