Acute promyelocytic leukemia (APL) is characterized by expression of promyelocytic leukemia (PML)/retinoic acid (RA) receptor α (RARα) protein and all-trans-RA-mediated clinical remissions. RA treatment can confer PML/RARα degradation, overcoming dominant-negative effects of this oncogenic protein. The present study uncovered independent retinoid degradation mechanisms, targeting different domains of PML/RARα. RA treatment is known to repress PML/RARα and augment ubiquitin-activating enzyme-E1-like (UBE1L) protein expression in NB4-S1 APL cells. We previously reported RA-induced UBE1L and the IFN-stimulated gene, 15-kDa protein ISG15ylation in APL cells. Whether the ubiquitin-like protein ISG15 directly conjugates with PML/RARα was not explored previously and is examined in this study. Transient transfection experiments with different PML/RARα domains revealed that RA treatment preferentially down-regulated the RARα domain, whereas UBE1L targeted the PML domain for repression. As expected, ubiquitin-specific protease 18 (UBP43/USP18), the ISG15 deconjugase, opposed UBE1L but not RA-dependent PML/RARα degradation. In contrast, the proteasomal inhibitor, N-acetyl-leucinyl-leucinyl-norleucinal, inhibited both UBE1L- and RA-mediated PML/RARα degradation. Notably, UBE1L induced ISG15ylation of the PML domain of PML/RARα, causing its repression. These findings confirmed that RA triggers PML/RARα degradation through different domains and distinct mechanisms. Taken together, these findings advance prior work by establishing two pathways converge on the same oncogenic protein to cause its degradation and thereby promote antineoplastic effects. The molecular pharmacologic implications of these findings are discussed. [Mol Cancer Ther 2008;7(4):905–14]