Bloom Syndrome (BS) is a human autosomal genetic disorder characterized by a predisposition to a variety of malignant tumors. The gene responsible for BS encodes a protein (BLM) consisting of 1417 amino acids with a nuclear localization signal in the C-terminal region, which is a member of the RecQ helicase family. We previously showed, using a yeast two-hybrid system, that BLM interacted with Ubc9, which is the conjugating enzyme of SUMO-1 (small ubiquitin-related modifier-1). In the present study, we exogenously expressed a green fluorescent protein-tagged Bloom syndrome protein, GFP-BLM, in human 293EBNA cells and found that it formed dots/rod-like structures associated with SUMO-1 in the nucleus. Deletion experiments indicated that the region from amino acids 238 to 586 of BLM is required for the formation of dots/rod-like structures associated with SUMO-1, and the DNA helicase domain, but not the helicase activity itself, slightly affected the formation and/or stability of these structures. Expression of a GFP-BLM which contained the 238-586 region, but lacked the C-terminal nuclear localization signal, resulted in localization to the cytoplasm without the formation of dots/rod-like structures and association with SUMO-1, indicating that these events occur only in the nucleus.