The ARHGEF5/TIM oncogene belongs to the Dbl family of guanine nucleotide exchange factors (GEFs) for Rho GTPases. It is well established that Rho-GEFs play an important role in tumorigenesis and metastasis through the activation of their substrates, the Rho GTPases. Little is known about ARHGEF5/TIM oncogene expression and cellular functions. Because of its localization close to the common fragile site FRA7I, which has been shown to be responsible for an inverted duplication of the 7q34-q35 region in breast carcinoma cells, we examined the expression of the ARHGEF5/TIM oncogene in normal and tumoral breast tissue. We report here the identification of five novel ARHGEF5/TIM alternative transcripts specifically expressed in breast tumors. These variant transcripts were characterized by the absence of one or several exons, all coding for the catalytic Dbl-homology domain and generating modified or truncated predicted variant proteins. The variant transcripts were predominantly expressed in breast carcinoma cell lines and in the most aggressive primary breast carcinomas, suggesting they may play a role in breast tumor progression. Moreover, we demonstrate that the expression of recombinant ARHGEF5/TIM protein in transfected COS-7 and NIH-3T3 cells generated a loss of actin stress fibers and the formation of membrane ruffles and filopodia. This pattern suggests that ARHGEF5/TIM activates Rac1, Cdc42 or RhoG rather than RhoA, as previously demonstrated in in vitro guanine nucleotide exchange assays. We anticipate that the activation of the ARHGEF5/TIM oncogene, possibly by the variant isoforms detected here, may play an important role in proliferative breast disease.