We report the full cDNA sequence encoding the human homologue of murine PA2.26 (T1α-2, podoplanin), a small mucin-type transmembrane glycoprotein originally identified as a cell-surface antigen induced in keratinocytes during mouse skin carcinogenesis. The human PA2.26 gene is expressed as 2 transcripts of 0.9 and 2.7 kb in several normal tissues, such as the placenta, skeletal muscle, heart and lung. Using a specific polyclonal antibody raised against a synthetic peptide of the protein ectodomain, PA2.26 was immunohistochemically detected in about 25% (15/61) of human early oral squamous cell carcinomas. PA2.26 distribution in the tumours was heterogeneous and often restricted to the invasive front. Double immunofluorescence and confocal microscopy analysis showed that PA2.26 colocalized with the membrane cytoskeleton linker ezrin at the surface of tumour cells and that its presence in vivo was associated with downregulation of membrane E-cadherin protein expression. Ectopic expression of human PA2.26 in HeLa carcinoma cells and immortalized HaCaT keratinocytes promoted a redistribution of ezrin to the cell edges, the formation of cell-surface protrusions and reduced Ca2+-dependent cell-cell adhesiveness. These results point to PA2.26 as a novel biomarker for oral squamous cell carcinomas that might be involved in migration/invasion. © 2004 Wiley-Liss, Inc.