Homologue of Slimb (HOS) is the substrate-recognizing component of the SCFHOS-Roc1 E3 ubiquitin protein ligase. This ligase mediates ubiquitination of the inhibitor of NF-κB transcription factor (IκB). We have found that HOS is highly expressed in a number of human cancer cell lines. The rates of the HOS gene transcription as well as HOS mRNA and protein levels were up-regulated in cells treated with mitogens or transfected with the inducers of mitogen-activated protein kinase pathway. Conversely, mitogen withdrawal strikingly reduced HOS levels during differentiation of mouse myoblasts. Activators of mitogen-activated protein kinase accelerated IκBα degradation and increased NF-κB transcriptional activity. Inhibition of HOS function via expression of dominant negative HOS (HOSΔF) initiated mouse myoblast differentiation and prevented Ras-mediated acceleration of IκBα degradation as well as NF-κB trans-activation and transformation of NIH3T3 cells. These data link the induction of HOS in proliferating cells with mitogen-signaling-dependent inhibition of cell differentiation and promotion of cell transformation.