UBE1L is the E1-like ubiquitin-activating enzyme for the IFN-stimulated gene, 15-kDa protein (ISG15). The UBE1L-ISG15 pathway was proposed previously to target lung carcinogenesis by inhibiting cyclin D1 expression. This study extends prior work by reporting that UBE1L promotes a complex between ISG15 and cyclin D1 and inhibited cyclin D1 but not other G₁ cyclins. Transfection of the UBE1L-ISG15 deconjugase, ubiquitin-specific protein 18 (UBP43), antagonized UBE1L-dependent inhibition of cyclin D1 and ISG15-cyclin D1 conjugation. A lysine-less cyclin D1 species was resistant to these effects. UBE1L transfection reduced cyclin D1 protein but not mRNA expression. Cycloheximide treatment augmented this cyclin D1 protein instability. UBE1L knockdown increased cyclin D1 protein. UBE1L was independently retrovirally transduced into human bronchial epithelial and lung cancer cells. This reduced cyclin D1 expression and clonal cell growth. Treatment with the retinoid X receptor agonist bexarotene induced UBE1L and reduced cyclin D1 immunoblot expression. A proof-of-principle bexarotene clinical trial was independently examined for UBE1L, ISG15, cyclin D1, and Ki-67 immunohistochemical expression profiles in pretreatment versus post-treatment tumor biopsies. Increased UBE1L with reduced cyclin D1 and Ki-67 expression occurred in human lung cancer when a therapeutic bexarotene intratumoral level was achieved. Thus, a mechanism for UBE1L-mediated growth suppression was found by UBE1L-ISG15 preferentially inhibiting cyclin D1. Molecular therapeutic implications are discussed. [Mol Cancer Ther 2008;7(12):3780–8]