Within the family of NADPH oxidases, Nox4 is unique as it is predominantly localized in the endoplasmic reticulum, has constitutive activity and generates H2O2. We hypothesize that these features are consequences of a so far unidentified Nox4-interacting protein. Interacting proteins were screened by quantitative SILAC-Co-immunoprecipitation in HEK293 cells stably overexpressing Nox4. By this technique, several interacting proteins were identified with calnexin showing the most robust interaction.