NF-kB transcription factor is activated upon ubiquitination and subsequent proteolysis of its inhibitor IkB. The phosphorylation-dependent ubiquitination is mediated by SCF E3 ubiquitin ligase. In this study, we identi®ed a novel murine F-box/WD40 repeat-containing protein, mHOS (a homologue of HOS/bTrCP2). mHOS eciently binds Skp1 protein (a`core' component of SCF ubiquitin ligase), and phosphorylated IkBa. We found that mHOS associates with SCF-ROC1 E3 ubiquitin ligase activity. We have also observed that mHOS is overexpressed in chemically-induced mouse skin tumors, and its overexpression (but not accelerated IkB phosphorylation) coincides with the accelerated degradation of IkB in vivo. The role of mHOS in the constitutive activation of NF-kB in skin carcinogenesis is discussed.