Deletions in the 8p21-22 region of the human genome are among the most common genetic alterations in prostate carcinomas. Several studies in di erent tumor tissues, including prostate, indicate that there are probably multiple tumor suppressor genes (TSGs) present in this region. To identify candidate TSGs on 8p22 a YAC contig spanning this region was assembled and YAC clones retro®tted with a selectable marker (neo) were transferred into rat prostate AT6.2 cells. Two overlapping YAC clones showed greatly reduced colonyforming e ciency, indicating they may carry a TSG. Two BAC clones encompassing the overlapping region also appeared to exert suppressive e ects on the growth of AT6.2 cells. Database searches for genes mapped to the critical region identi®ed a gene known as FEZ1 (LZTS1) as a potential candidate suppressor gene. Subsequent experiments showed that over-expression of LZTS1 cDNA inhibited stable colony-forming e ciencies of AT6.2, HEK-293 and LNCaP cells. In contrast, LZTS1-transfected Rat-1 and RM1 cells were growthstimulated. Database searches also identi®ed additional isoforms of the LZTS1 mRNA, as well as LZTS1 protein domains reminiscent of those found in transcription factors. Together these data suggest that the LZTS1 gene is involved in the regulation of cell growth and its loss of function may contribute to the development of prostatic carcinomas, as well as other cancers. Oncogene (2001) 20, 4169 ± 4179.