PMEPA1 (prostate transmembrane protein, androgen induced 1) is a multifunctional transmembrane protein that serves as a critical negative regulator of TGF-β signaling in solid tumors. The protein exists in five distinct isoforms (a-e) with differential functional properties 1. PMEPA1 inhibits TGF-β pathway signaling through direct interaction with SMAD2 and SMAD3, competing with ZFYVE9 to prevent SMAD-mediated transcriptional activation 2. Additionally, PMEPA1 promotes ubiquitin-mediated degradation of the androgen receptor (AR) through recruitment of NEDD4, thereby modulating androgen signaling 3. PMEPA1 expression is upregulated in 87% of renal cell carcinomas and in various solid tumors including prostate, colon, breast, lung, and ovarian cancers 4. In hepatocellular carcinoma (HCC), PMEPA1 overexpression (18% of cases) associates with TGF-β pathway activation, immune exhaustion, and poor prognosis, with mouse models demonstrating that MYC+PMEPA1 overexpression promotes HCC development 5. PMEPA1 expression also correlates with hypoxia-inducible factor-1 (HIF-1) activity, linking it to tumor microenvironmental adaptation 2. High PMEPA1 expression predicts poor outcomes across multiple cancer types and correlates with immune checkpoint gene expression and tumor mutational burden 6. Recent evidence indicates that truncating PMEPA1 variants associate with Loeys-Dietz syndrome, revealing disease relevance beyond cancer 7.