Nedd4-binding partner-1 (N4BP1) is a developmentally expressed protein interactor and monoubiquitylation substrate of Nedd4, a HECT domain-containing ubiquitin ligase that mediates ubiquitylation and proteasome degradation of target proteins and may have a fundamental role to play in embryonic processes. N4BP1 can undergo Nedd4-mediated polyubiquitylation and proteosomal degradation. N4BP1 also functions as a negative regulator of AIF4 (also known as ITCH or Atrophin-1), an E3-ubiquitin ligase, by interacting with the second WW domain of AIF4 and impairing ubiquitination of substrates.