Cyclin E1 regulates the initiation of the S phase program in the mammalian cell division cycle. In normal cells, cyclin E1 protein expression is tightly controlled through a combination of transcriptional and proteolytic regulatory processes. However, in many types of human tumor, cyclin E1 expression is frequently dysregulated, including overexpression, nonperiodic expression relative to cell division, and generation of low molecular weight (LMW) derivatives. LMW derivatives of cyclin E1 have been proposed to be generated by the in vivo proteolytic cleavage of the full-length cyclin E1 protein by a yet to be identified tumor-specific protease. Recently, it was suggested that overexpression of full-length or LMW derivatives of cyclin E1 are independent variables associated with poor outcome in patients with breast cancer. However, we have extensively analyzed cyclin E1 protein expression in primary breast tumors and breast tumor-derived cell lines and found that the ability to detect LMW derivatives of cyclin E1 correlates only with the level of cyclin E1 protein. When cyclin E1 levels on Western blots are normalized, LMW derivatives of cyclin E1 were observed at roughly equal levels in all primary breast tumors, breast tumor-derived cell lines, immortalized nontransformed human mammary epithelial cells, and normal breast tissue. Therefore, the detection of LMW derivatives of cyclin E1 is likely a function of cyclin E1 protein levels, and the activity of the proteolytic machinery responsible for their generation is not a tumor-specific property.