F box proteins are an expanding family of eukaryotic proteins characterized by an approximately 40 amino acid motif, the F box. Some F box proteins have been shown to be critical for the ubiquitin mediated degradation of cellular regulatory proteins. In fact, F box proteins are one of the four subunits of ubiquitin protein ligases, called SCFs. SCF ligases bring ubiquitin conjugating enzymes to substrates that are specifically recruited by the different F box proteins. A large family of mammalian F box proteins has recently been identified and classified into three groups based on the presence of either the WD 40 repeats, the leucine rich repeats, or the presence or absence of other protein protein interacting domains. The FBXW2 gene product, the second identified member of the F box gene family, contains multiple WD 40 repeats.