WWP1 is an E3 ubiquitin ligase and belongs to a family of NEDD4-like proteins. WWP1 contains 4 tandem WW domains and a HECT (homologous to the E6-associated protein carboxyl terminus) domain. WW domain-containing proteins are found in all eukaryotes and play an important role in the regulation of a wide variety of cellular functions such as protein degradation, transcription, and RNA splicing. The HECT domain of WWP1 has been implicated in regulating the localization and stability of p53 – inhibition of WWP1 results in a decrease in p53 expression, whilst WWP1 mediated stabilization of p53 appears to be associated with an accumulation of cytoplasmic p53. WWP1 also negatively regulates the TGF beta tumor suppressor pathway by inactivating its molecular components (SMAD2, SMAD4 and TGFbetaR1). WWP1 has been implicated in both breast and prostate cancers.