The proto-oncogene Frat1 was originally identified as a common site of proviral insertion in transplanted tumors of Moloney murine leukemia virus (M-MuLV)-infected Emu-Pim1 transgenic mice. Contrary to most common insertion sites implicated in mouse T cell lymphomagenesis, retroviral insertional mutagenesis of Frat1 constitutes a relatively late event in M-MuLV-induced tumor development, suggesting that proviral activation of Frat1 contributes to progression of T cell lymphomas rather than their genesis. To substantiate this notion we have generated transgenic mice that overexpress Frat1 in various organs, including lymphoid tissues. Frat1 transgenic mice develop focal glomerulosclerosis and a nephrotic syndrome, but they do not exhibit an increased incidence of spontaneous lymphomas. Conversely, these mice are highly susceptible to M-MuLV-induced lymphomagenesis, and Frat1/Pim1 bitransgenic animals develop lymphomas with increased frequency compared to Pim1 transgenic littermates. These data support a role for Frat1 in tumor progression.