['Portal vein tumor thrombus (PVTT) has previously been demonstrated to correlate with poor prognosis of hepatocellular carcinoma. Approximately 50-80% of HCC is accompanied by portal or hepatic vein invasion. The underlying mechanisms of PVTT development remain unclear. This study aimed to elucidate the role of miR-135a in PVTT tumorigenesis.', 'In the present study, we investigated the expression of microRNAs and mRNAs in PVTT tissues using advanced microRNA and cDNA microarray techniques. MicroRNA (miR)-135a was noted to be highly over-expressed in PVTT and the cell line CSQT-2 and was selected for further study. We characterized the function of miR-135a in vitro and in vivo. We also analyzed the clinical relevance of miR-135a in relation to the prognosis and survival of HCC patients with PVTT.', 'Our analyses found that the miRNA and mRNA expression profiles of PVTT were distinct from the parenchyma tumor. Overexpression of miR-135a favors invasive and metastatic behavior in vitro. Furthermore, in a CSQT-2 orthotopic transplantation nude mouse model, blockade of miR-135a significantly reduced PVTT incidence. We also found that miR-135a was transcribed by forkhead box M1 (FOXM1), and metastasis suppressor 1 (MTSS1) was identified as the direct and functional target of miR-135a. Additionally, the cohort analysis revealed the relevance of miR-135a with respect to the prognosis and survival of HCC patients with PVTT.'] Does microRNA-135a contribute to the development of portal vein tumor thrombus by promoting metastasis in hepatocellular carcinoma?