Cyclin E, a regulatory subunit of cyclin dependent kinase-2, is thought to be rate limiting for the G1/S transition during the mammalian cell cycle. Previously, we showed severe alterations in cyclin E protein expression in human mammary epithelial cell lines and in surgical material obtained from patients with various malignancies. To understand the functional basis of these alterations we analyse here the regulation of cyclin E in breast cancer cells. We find that while cyclin E protein and its associated kinase activity in normal cells are cell cycle regulated, in tumor cells it remains in an active complex throughout the cell cycle. We also analysed cyclin E for possible deletions which could result in its constitutive function and found two novel truncated variants in its coding region. These variant forms of cyclin E were detected in several normal and tumor cell lines and tissue specimens. However, Western blot analysis indicated that only the multiple isoforms of cyclin E protein were expressed in tumor but not the normal tissue specimen, suggesting post transcriptional regulation of cyclin E. Lastly, in vitro analyses indicated that these truncated variant forms of cyclin E are biochemically active in their ability to phosphorylate histone H1. Collectively these observations suggest the presence of more than one form of cyclin E mRNA in all cells, normal and tumor. Once translated in tumor cells, the protein products of these truncated forms could give rise to a constitutively active form of cyclin E containing complexes.