PML protein is mainly present inside the nucleus in the form of PML nuclear bodies (PML-NB) and serves as anchoring point for various sumoylated transcription factors and regulators. It mediates activities, such as transcriptional regulation, antiviral defense, DNA replication, DNA repair, telomere lengthening, chromatin organization, cell cycle control, senescence, apoptosis, and tumor suppression. PML-NB is associated with acute promyelocytic leukemia (APL). All known isoforms of PML have conserved N-terminal RBCC region comprised of a RING finger domain followed by two B-boxes and coiled-coil region. The RING domain has been shown to possess SUMO E3 ligase activity. Surprisingly, only the structure of the RING domain was determined in 1995 and no information on the structure of other part of the molecule is known. We have determined the solution NMR structure the RING domain and the B1 box. We have also investigated the interactions of these domains with Ubc9, SUMO and SUMO substrates by NMR and SPR. These results will be presented in poster.