Transforming growth factor (TGF)-β1 induces fibroblast transdifferentiation to myofibroblasts, a process that requires the involvement of integrin-mediated signaling and focal adhesion kinase (FAK). FAK-related non-kinase (FRNK) is known for its role in inhibiting integrin-mediated cell migration; however, its role in myofibroblast differentiation has not been defined. Here, we report that FRNK abrogates TGF-β1-induced myofibroblast differentiation in vitro and in vivo. TGF-β1 can induce α-smooth muscle actin (α-SMA) expression in the presence or absence of FAK; however, TGF-β1-induced α-SMA expression is reduced (∼73%) in FAK-deficient fibroblasts. Although both ERK and p38 MAPK activation is required for maximal TGF-β1-induced α-SMA expression, ERK is the major signaling intermediate in cells that express FAK. In contrast, p38 MAPK is the dominant mediator of TGF-β1-induced α-SMA expression in FAK-deficient cells. FRNK overexpression blocks TGF-β1-induced ERK or p38 MAPK activation in the presence, and surprisingly, in the absence of FAK. The loss of FRNK was tested in vivo during experimentally induced pulmonary fibrosis in mice. FRNK knock-out mice have a greater increase in α-SMA-expressing cells in response to a pulmonary fibrotic stimulus in vivo, as compared with congenic wild type mice. This is the first time that FRNK loss has been shown to modify the pathobiology in any animal disease model. Together, the data demonstrate that FRNK negatively regulates myofibroblast differentiation in vitro and in vivo. These data further suggest that modulation FRNK expression may be a novel avenue for therapeutic intervention in tissue fibrosis. © 2008 by The American Society for Biochemistry and Molecular Biology, Inc.