Skp2 is an F-box protein involved in the ubiquitination and subsequent degradation of the cyclin-dependent kinase (Cdk) inhibitor p27. Skp2 has been reported to be overexpressed in a variety of cancer types and to correlate with poor prognosis. We have identified a novel isoform of Skp2 we named Skp2B, which differs from Skp2 only in the C-terminal domain and unlike Skp2 localizes to the cytoplasm. Here, we describe the relative expression of both Skp2 and Skp2B in breast cancer cell lines and in primary breast cancers using quantitative real time RT-PCR. We show that Skp2B mRNA is expressed 10-fold less than Skp2 mRNA in the immortalized but non-transformed breast cell line, 184B5. However, Skp2B is overexpressed as frequently as Skp2, and to higher levels than Skp2 in breast cancer cell lines and primary cancers. Further, we show that cytoplasmic staining is frequent in primary breast cancers. In addition, we found that xenografts expressing Skp2B grow faster than xenografts expressing low levels of Skp2B, and that this effect is independent of p27 degradation. These findings therefore suggest that Skp2B overexpression is also observed in breast cancers and identify Skp2B as a putative oncogene.