The standard form of cell adhesion glycoprotein CD44 is a metastasis suppressor in prostate cancer. However, we previously showed by RT-PCR and Western blotting that cancer overexpresses unique CD44 variant v7-v10 isoforms. Muc18 is another cell adhesion marker reportedly overexpressed by prostate cancer. Matched frozen section-confirmed tumor and benign tissues were harvested from 10 prostatectomy specimens and tumor was microdissected from two lymph node metastases. Tissues were homogenized for RNA preparations, and RT-PCR was performed for the CD44v7-v10 sequence. In cultured prostate cancer cells, we caused RNA interference against CD44v9 and/or Muc18. We used PC3M cells and a derivative cell line called G s a, that constitutively expresses this G-protein and is more invasive. Lipofection was performed for a green fluorescent protein plasmid and for two 22-mer DNA fragments, cloned into a plasmid expression vector to generate hairpin, interfering dsRNA. Assays for invasion into Matrigel, a basement membrane matrix, were performed in 4-5 experiments. RT-PCR demonstrated expression of a 608 bp band representing CD44v7-v10 or a 638 bp band of Prostate cancer remains a public health problem of enormous magnitude; it is the second most common cause of cancer fatality among North American men and the most common in Great Britain. Only a minority of all cases invade locally and metastasize, thus requiring altered expression of cell adhesion molecules to allow tumor cell detachment, migration through a stromal matrix, and lymphovascular invasion.CD44, a family of transmembrane glycoproteins involved in homotypic cell, cell-matrix, and cellcytoskeletal interaction, holds promise as a prostate tumor marker. The extracellular domain of CD44 binds numerous matrix substituents: hyaluronic acid, heparin-affinity growth factors, vascular endothelial growth factor, p185 HER2 , epidermal growth factor, and hepatocyte growth factor. Its intracellular domain binds to ezrin, radixin, moesin and merlin, which interact with the cytoskeleton; notably, neutral endopeptidase 24.11 competes with CD44 for this interaction. 1 CD44 also binds directly to the cytoskeletal substituent ankyrin, thus determining cell and tissue architectural form. 2 CD44 is an oncodevelopmental protein, with roles in