Homo-oligomerization via a coiled-coil (C-C) domain has been shown to be necessary for the promyelocytic leukemia (PML)-retinoic acid receptor-alpha (RARalpha) fusion protein to acquire oncogenic potential in acute promyelocytic leukemia. We show here that PML(DeltaC-C)-RARalpha, which contains a deletion in its C-C domain, is neither localized as characteristic microspeckles nor modified by small ubiquitin-like modifiers (SUMO). The absence of sumoylation of the DeltaC-C mutant was due to the lack of binding to Ubc9, a SUMO conjugation enzyme. The integrity of RING finger domain was also needed for both sumoylation and microspeckle formation. In GAL4-DNA tethering assays, the DeltaC-C mutant completely lost the inhibitory effect on retinoic acid (RA)-mediated transactivation. Furthermore, the expression of CD14 in U937 cells expressing the DeltaC-C mutant in response to vitamin D3 was markedly higher than in cells expressing PML-RARalpha. However, the RA-mediated induction of C/EBPbeta in cells expressing the DeltaC-C mutant was comparable to that of control cells. Thus, our results suggest that the C-C domain-associated functions of sumoylation, localization as microspeckles, and the inhibition of monocyte differentiation all contribute to the oncogenic activity of PML-RARalpha.