Purpose: By characterizing a complex chromosome rearrangement involving 6q and 17p in melanoma cell line UACC-930, we isolated a candidate tumor suppressor gene at 6q21, named prenyl diphosphate synthase subunit 2 (PDSS2), which was interrupted by an inversion breakpoint. The purpose of this study was to determine the tumor-suppressive potential of PDSS2 in the development of melanoma. Experimental Design: To isolate the rearranged 6q in UACC-930 cells, a bacterial artificial chromosome clone (RP1-67A8) covering the breakpoint at 6q21 was digested with HindIII and each DNA fragment was used as the probe for the breakpoint in Southern blotting. The HindIII fragment probe covering the breakpoint was then used to screen an EcoRI-digested DNA library generated from UACC-930. To characterize the tumor-suppressive potential of PDSS2, PDSS2 was stably transfected into a highly tumorigenic melanoma cell line, UACC-903. The tumor-suppressive function of PDSS2 was shown by both in vitro and in vivo assays. The differential expression of PDSS2 in benign nevi and primary melanoma samples was also studied. Results: Down-regulation of PDSS2 was observed in 59 of 87 (67.8%) primary melanomas, which was significantly higher than that in benign nevi (7 of 66, 10.6%; P < 0.001). In addition, an overexpression of the PDSS2 in UACC-903 cells could inhibit tumor cell growth, decrease the colony-forming ability in soft agar, and totally abrogate the tumorigenicity of UACC-903 in nude mice. Conclusions: Our results support the proposal that PDSS2 is a novel tumor suppressor gene that plays an important role in the development of malignant melanoma.Melanoma is the most serious type of skin cancer (1). Genetic alterations in melanoma have been widely studied and the deletion of 6q is one of the most common chromosomal changes (2 -4). The reversion of the tumorigenic phenotype of melanoma cells by introducing a normal human chromosome 6 provides the first biological evidence that chromosome 6 contains a tumor suppressor gene (TSG) associated with melanoma pathogenesis (5). The abnormality of 6q has also been detected in other malignancies, including malignant lymphoma (6), ovarian (7), and prostate carcinomas (8).Several studies showed that 6q21 is the most frequent deleted region, suggesting the existence of a TSG at 6q21 (2, 3, 9). Although a couple of candidate TSGs at 6q, including AIM1 at 6q21 (10) and SOD2 at 6q25.3 (11), has been studied, the underlying mechanisms of these genes in melanoma development are unclear. In addition, the tumor-suppressive role of SOD2 is controversial. Miele et al. (12) found that SOD2 could not suppress tumorigenicity or metastasis of human melanoma C8161 cells. Therefore, more effort in looking for new candidate TSGs at 6q related to melanoma pathogenesis is justified.In our previous study, we found a reciprocal-like translocation between 6q21 and 17p13 in the melanoma cell line UACC-930 (9). The translocation was further characterized by chromosome microdissection and fluorescence in situ h...