The development and progression of malignancies is a complex multistage process that involves the contribution of a number of genes giving growth advantage to cells when transformed. The role of transforming growth factor-β (TGF-β) in carcinogenesis is complex with tumor-suppressor or prooncogenic activities depending on the cell type and the stage of the disease. We have previously reported the identification of a novel WD-domain protein, STRAP, that associates with both TGF-β receptors and that synergizes with the inhibitory Smad, Smad7, in the negative regulation of TGF-β-induced transcription. Here, we show that STRAP is ubiquitously expressed and is localized in both cytoplasm and nucleus. STRAP is up-regulated in 60% colon and in 78% lung carcinomas. Stable expression of STRAP results in activation of mitogen-activated protein kinase/extracellular signal-regulated kinase pathway and in down-regulation of the cyclin-dependent kinase inhibitor p21Cip1, which results in retinoblastoma protein hyperphosphorylation. In addition, we have observed that Smad2/3 phosphorylation, TGF-β-mediated transcription, and growth inhibition are induced in STRAP-knockout mouse embryonic fibroblasts compared with wild-type cells. Ectopic expression of STRAP in A549 lung adenocarcinoma cell line inhibits TGF-β-induced growth inhibition and enhances anchorage-independent growth of these cells. Moreover, overexpression of STRAP increases tumorigenicity in athymic nude mice. Knockdown of endogenous STRAP by small interfering RNA increases TGF-β signaling, reduces ERK activity, increases p21Cip1 expression, and decreases tumorigenicity. Taken together, these results suggest that up-regulation of STRAP in human cancers may provide growth advantage to tumor cells via TGF-β-dependent and TGF-β-independent mechanisms, thus demonstrating the oncogenic function of STRAP.